See Install an older version of Firefox

It may well be worth checking for the plugin becoming compatible with Firefox 5. Most active plugins will probably very quickly be made to work with the latest version of Firefox.

That allowed me to download 3.6. The add-on that I have will only work with 4.

In that case (which is rather unusual an extension working with firefox 4 but not 3.6 - the converse tends to be more likely) you need Firefox4 and I see your difficulty with the links provided.

Hopefully this will be quickly rectified, but if you are in a hurry try another link (assuming windows and US) ftp://ftp.mozilla.org/pub/firefox/releases/4.0rc2/win32/en-US/

You will need to download the files and then run the saved .exe file.

I note currently Firefox 5 is released but the all older page http://www.mozilla.com/en-US/firefox/all-older.html is indeed only fx3.6, maybe it should have updated to fx4. Probably this will happen over the next few days. It should also allow access to other versions. (I will comment elsewhere so that this is investigated)

EDIT Apparently this is a consequence of the new faster release cycle. Read the article already linked to in this thread. (I commented in the article discussion section ). Remember the faster release cycle also has relatively user friendly Beta and Aurora channels. (pity the easy UI user switching did not go ahead)

I have also been reminded about the Add-on compatibility reporter. that you may wish to try.

What is the add-on that is causing a problem, is it an ordinary one downloaded from mozilla, or created by your work IT department, if the latter, contact them, if the former, what is it ?

This new link worked perfectly. Thanks.

edited
Please stop linking to Firefox 4 - it is not supported any longer. If some one wants to revert to an older version, Firefox 3.6.19 is the version to recommend.
http://www.mozilla.com/en-US/firefox/all-older.html

moderator request