That issue can be caused by corrupted cookies.

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

As I said I've cleared cache and removed cookies. Anyways I did it again and no luck, still the same error:

Bad Request

Your browser sent a request that this server could not understand. Apache/2.2.16 (Debian) Server at ylilauta.fi Port 80

It's not just ylilauta.fi, any other site does this as well

If clearing the cookies doesn't help then it is possible that the file cookies.sqlite that stores the cookies is corrupted.
Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete other present cookie files like cookies.sqlite-journal in the Firefox Profile Folder in case the file cookies.sqlite got corrupted.

• http://kb.mozillazine.org/Cookies

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Okay, so I tried deleting cookies.sqlite. Didn't work out. Then I tried safe mode, and it did work. Weird because even after re-installing it did not work without all the addons. Anyways, now I need to find out which of the addons is conflicting with uploading, any idea how I could identify the one that is causing problems?

Oh well, I cleared out the list and only left the necessities. Thanks for the support, everything seems to be working out now

This worked for me also. A Firefox add-on ( not sure which yet) was preventing an extension from being installed into Joomla

Hi, I had a similar problem uploading a file. I followed your suggestions and I found this plug-in: Microsoft DRM 9.0.0.4503 Network Object After after disabling that plug-in, it works for me

Worked for me as well, I had an issue where i was being blocked from viewing only 1of 4 php apps(not wp plugins), on my wordpress website. The website was also viewable with no errors. I only removed the cookies for that domain and that was enough to fix it thanks!

Check your permalinks of your web site: make sure are configured correctly; and/or reconfigure them. If you are using "custom permalinks" consult with Wordpress.org how to use custom permalinks.