Guys i think i solved this issue :D All what you have to do is go to about:config then click "I'll be careful i promise" now right click somewhere then click New>String and name it: xpinstall.enabled and set the value to true

All the downloads should work now

Where is about:config? Didn't find it. Please help. Thanks!

Type it in adress o-o

Tried that, but no luck :( Started happening on Firefox 7.0, on both WinVista and Win7.

I replied to an older post, but since this is new and fresh i'll reply here too:

I am using Windows XP and Firefox 7 and I am getting this. I added the xpinstall:enabled line into the about:config but that's not helping. I still get the same error. Would really like to fix this soon.

It should also be noted that I was able to successfully install adblock and Lastpass. Since then I have not been able to install anything else.

Like mjulian211, I am also replying to a duplicate post.

Also getting this error. Running Ff 7.0 in Windows XP Home SP3. PartyPengy's solution did not work for me. I even tried restarting Ff after the about:config edit, even though it didn't say you had to. No joy.

Edit: Okay, mha007's workaround from this thread:

https://support.mozilla.com/en-US/questions/855759#answer-253445

worked for me, except in Ff7 the link says, "+ Add to Firefox." Also, to open it, you have to click on the Ff menu button, hover over "New Tab," and then click "Open File...."