can you try changing network.http.spdy.enabled to false temporarily, restart the browser and see if this does make any difference?

I gave it a try but that didn't help either.

ok then please turn the setting back to the default value (since it might have other repercussions).

is this only happening when you're trying to install one certain addon or for all of them? please also try to clear the cache and cookies...

Ok I set it back to the default value. I also tried clearing cache and cookies and restarting the browser, still didn't work.

Can you download the extension's XPI file from the Add-ons site and save the file to your computer (Desktop)?

• Right click the install (Add to Firefox) button and choose "Save Link As".

Then you can install the extension via File > Open File of Firefox > New Tab > Open File.

• http://kb.mozillazine.org/Installing_extensions

No I tried that as well. It gives me the message: "C:\Users\hp\Desktop\addon-3006-latest.xpi could not be saved, because the source file could not be read.

Try again later, or contact the server administrator."

It is possible that your anti-virus software is corrupting the downloaded files or otherwise interfering with downloading files.
Try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading work.

See "Disable virus scanning in Firefox preferences - Windows"

• http://kb.mozillazine.org/Unable_to_save_or_download_files

I disabled Real-Time virus scanning and tried Right Click + Save Link As and this time I got this message: "The download cannot be saved because an unknown error occurred.

Please try again."

And I tried numerous times. This problem seems unresolvable!

I just tried again and this time choosing a destination file worked and I disabled Real-Time virus scanning before clicking save but I got the same first message I posted.

Any other options?

There is a temporary solution you can use it until this problem is resolved.

1- you have to download the api file for your add-on to your PC - You may use another browser such as Chrome to download it.

2- In the menu at the top of the Firefox window, click the Tools menu, and select Add-ons. The Add-ons window is displayed.

3- To add the downloaded add-on to the list of available add-ons, drag and drop the file into the Add-ons window. The Add-on is added to the list.

4-The installation process should begin.