-> Tap ALT key or press F10 to show the Menu Bar

-> go to Tools Menu -> Clear Recent History -> Time range to clear: select "EVERYTHING" -> click Details (small arrow) button -> place Checkmarks on Cookies, Cache, Site Preferences -> click Clear Now

-> go to Tools Menu -> Options -> Security -> place Checkmarks on:

1) Warn me when sites try to install add-ons 2) Block reported attack sites 3) Block reported web forgeries

-> go to Tools Menu -> Options -> Advanced -> Network -> Offline Storage (Cache): click Clear Now button

-> Click OK on Options window -> Restart Firefox

Check and tell if its working.

Same problem over here. Not sure if it is related: I created a new profile (via profile manager) and did try to download again some of my existing add-ons in the new profile. All addons.mozilla.org add-ons report the connection failure. I did return back to my old profile, same problem over there. @mha007 : followed your advice - but the issue is still there...

Unable to install add-ons

https://support.mozilla.com/en-US/kb/Unable%20to%20install%20add-ons

Check and tell if its working.

mha007, tried your steps, and it doesn't work. I looked at the support page, and nothing there is pertaining to my problem. The error I explained to above is my issue, nothing more.

Although, after changing my settings as perscribed by you in your first comment. I know get the prompt to allow it to download. It looks as if it's going to go through, then it goes right back to the "This add-on could not be downloaded because of connection failure" message.

Also, glad to know I'm not the only one with the error. Hopefully someone would be so kind to solve our dilemma.

I have the same problem. I'm using Windows 7. And I can't add add-ons. It just tells me: "This add-on could not be downloaded because of connection failure"

Seriously when are you going to fix it :/ This is HUGE bug. I have the same problem on Windows 7

it seems FF's fix is outdated. on the https://support.mozilla.com/en-US/kb/Unable%20to%20install%20add-ons page it says to look for xpinstall.enabled under about:config, but it appears xpinstall.enabled does not exist for Windows 7

neither it does exist for Firefox 5/winxp. Unfortunately, other recommendations in referred kb didn't help. Other suggestions, before going for a reinstall?

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

nice. gonna try that!

A work around is right-click Click to Download link -> click Save Link As... -> Save the file on your computer -> when it has Downloaded then go to File Menu -> click Open File... -> select the saved file of your add-on (it will be saved as .xpi file) and hit Enter key

Now it should be installed. Check and tell if its working.

partypengy's fix worked for me! thanks!

partypengy's fix worked for me too ! thanks Still a weird problem: checked on other PC (also XP/FF 5.0) which has no problem while it has no atttribute xpinstall.enabled as well.

Hmm.... maybe that bug is only on Windows 7 And you're welcome everyone (:

By the way, if you delete the value later it still works

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 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...."

I had this problem and it turned out that the date on my computer was set incorrectly. This was preventing Mozilla's site from being "trusted" due to the date on my computer being outside of the range of the beginning and ending dates of Mozilla’s security certificate. So the Add-On would not download.

Check the date on your computer as another possible solution.

I tried that but it doesn't seem to work!

It works for me, but you need to restart Firefox then try to install the add on again. Hope this will help :-)