You can check the network.http.* prefs on the about:config page.
Reset user set (bold) network.http prefs to the default value via the right-click context menu -> Reset.

Check at least:

• network.http.accept-encoding (default: gzip,deflate)
• http://kb.mozillazine.org/network.http.accept-encoding

I have did everything that have been recommended and I am still receiving “The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression.” message.

So, all network.http prefs have the default value?

Reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Press and hold Shift and left-click the Reload button.
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (MAC)

A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls

You can try to reset Firefox.

• https://support.mozilla.org/kb/reset-firefox-easily-fix-most-problems

If you reset Firefox then a new profile is created and some of your data (bookmarks, passwords, cookies, form data) is automatically imported and your current profile will be moved to the desktop (Old Firefox Data).

You can recover more data from the old profile (be cautious with doing that).

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile

I'M HAVING THE SAME PROBLEM EVERYTIME YOU DO AN UPDATE I CAN'T GET ONTO SEVERAL PAGES LIKE AMAZON AND THIS DID HAPPEN OVER XMAS ... I TRIED EVERYTHING BUT RE-INSTALL WINDOWS 7 ... OLDER VERSIONS ... WHAT I DID NOTICE IS THAT THE COOKIE SETTINGS WONT SAVE .... IF THERE'S GOING TO BE THIS MANY UPDATES I'M SURE EVERYONE IS GOING TO FIND ANOTHER BROWSER .... THERE'S AN UPDATE ALMOST EVERY OTHER WEEK

I googled the problem and I found the answer. Yesterday I bought RealPlayer Plus and the Speedbit Video Accelerator.

Someone suggested that the Speedbit Video Accelerator IS the cause of the problem, so I killed the process and.... EVERYTHING is fine ! Since I paid for the Accelerator, I wil not uninstall it, but I will prvent it form loading at start-up. Hopefully, with a new Firefox version it will work.

Here is the original post I found on the net:

" The problem turned out to involve something called Speedbit Video Accelerator which comes as a bundled option with RealPlayer (yes I know). When in Firefox, in my setup, no access to sites (not just ARRSE) which have file compression such as gzip. Unwanted program removed, problem solved."

I have both Speedbit and Real Player and both worked fine for video downloads on older versions of Firefox. Both work OK currently with Microsoft IE8. So the problem would appear to be with latest version of Firefox. Until I upgraded to F/fox 19 I would get "Hooray your Firefox is up to date" - close tab then onto homepage. Now with F/fox 19 I get this compression error message and I cannot even use search bar because of error message.. If it cannot be solved then it's goodbye Firefox hello MS Internet Explorer.

I finally got a RESPONSE and I tried all that ... I SAW THIS PAGE !!!! ITS YOU NOT MY COMPUTER NOT AMAZON AND NOT ANY OF THE OTHER PAGE ... FACEBOOK WORKS AND SO DOES YOUTUBE ... I found my Bank worked but I'm not LOGGED onto My Bank for the more than 5 mins I just need to verify a couple things and its done ... I can't shop on Amazon and its Frustrating ... I love all the Add-Ons but whats the point if I can't use them

Understand your frustration TonyVac - but by direct comparison I have confirmed that Speedbit and Real Player downloads work perfectly on IE8 as does everything else including Amazon etc. .I can use either IE8 or F/Fox on my PC but up till now have preferred the latter.I have both S/Bit and R/Player Plus 16 as separate systems and note raduf that S/Bit Accelerator doesn't automatically come with Real Player - it used to.I cannot even do much with my TalkTalk homepage since if I even use Google search I get this error compression message. So the problem I repeat is with current version(s) of Firefox.

I have same problem and had speedbit as part of real player plus. uninstalled and reinstalled (you can download speedbit separately tho it comes withthe plus.;) That did not eliminate problem, in fact when they were uninsalled still had the problem tho less cpu usage. Firefox seems top be using a lot more cpu than chrome or IE8. The only thing that has worked for me to some extent is resetting the preference on the network.http.* prefs after typing in about:config in URL bar and changing values from default value of gzip,deflate to "true"-you have to type that in. Mozilla help keeps tellingme and otheres not to do that and to reset to the default and clear the caches but that does not help. In fact it was so f$#*ed up that when I tried to type about:config in the Firefox url it kept doing a Google search for "about:config" and I could not get to the page. I had to type it in and when the suggested searches popped up under the URL bar I hit delete on all of them until there were none left and then when I pasted it in it went there. At least when Real Networks has a buggy app they fix it and even give you live customer support (at least if you bought the "plus". When I searched the web for 'Content Encoding Error' The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression. Please contact the website owners to inform them of the problem." I saw posts going all the way back to 2008. All compression issues related to Firefox and no other browsers. I am using 19.0.2 & I do not know when it was put on with automatic updates which are not in control panel add remove programs so you cannot remove updates (which when I did not like the way IE8 worked with Links bar and removed it I had IE7-I do not know what I would get if I removed Firefox-probably no Mozilla product.) I think the last line of the error message is great: "Please contact the website owners to inform them of the problem."---that is google, almost every site-and the nerve of some geek to pass the blame onto all these websites when it is a browser issue is just unmitigated BS. Predecessor Netscape did not have that cavalier attitude and their products were not as buggy and you only had a new release every year or two. Their only bug problems were Microsoft initiated ones when it was trying to kill netscape just as it did try with real player-making the Windows media player the default even dfor .ra files which it could not play. The sentence about contacting the website is about just as bad.

 I would also ask why it is when there is a popular app such as real lplayer with real downloader that is working fine with Firefox and I have to think a lot of people are downloading .flv files and .mpegs from you tube and all kinds of other sites Mozilla messes with an update or new version (Why are they already on version 19 when IE is less than half that (9) and has been around a lot longer) that will not work with these apps but either does not workl to show the download button or allow a right click download or uses yup so much cpu to make  the computer go so slow you think you are goingt o uninstall Firefox and make IE, Chrome or Safari the default browser?   All my problems startede after I had a message "you must restart your computer for the 'Firefox update dto take effect' or something close to that, so I think it likely was a wholenew version which I did not even know I downloaded with the automatic updates and whatever it was  could  not make an intelligent decision as to whether to put it on my computer nor remove it from condtrol panel since updates are not in add/remove programs and when you click modify/remove button the only thing available is remove.  Sorry about going on and on, but I think the developers think they are doing us a favor and can be cavalier with us, such as telling us to tell the website about the compression proble!  They  or someone or several remind me of house  talking to a patient.  No I am computer literate and I do not need a geek to have a dgood bedside manner with me with his/her application, but House may have yelled at his patiens and been soewhant disingenuous with them, but he did not make many long term mistakes but quickly rectified them even if he plamed another MD or RN, but he did not blame the medical people in China, Iran or the DPRK for the issue. Thomas J McKeon, Indianapolis

If you want to change the network.http.accept-encoding pref then at last change it to a valid value and e.g. remove the gzip and only leave the deflate part or use an empty string. A (Boolean) value of true doesn't make any sense for this string pref.

• network.http.accept-encoding (default: gzip,deflate)
• http://kb.mozillazine.org/network.http.accept-encoding

Also make sure that your security software (firewall, anti-virus) isn't causing problems by modifying HTTP response headers.

Try to boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.