I usually update my Firefox as soon as possible but often, especially after installing the new version 5 on a Ubuntu 11.04 platform, I had to lose many add-ons that I previously selected and used daily. To avoid this problem I found some tips on the web and I tested all of them with different results.
– unzip the .xpi file with Archive Manager and open install.rdf using a Text Editor;
– find this line: em:maxVersion and change the value from e.g. 4.0.* to 5.0.*;
– click on icon in the Firefox Add-ons Manager, select “Install Add-on From File…” and follow the visual instructions.
Unfortunately this solution didn’t work always during my tests
– open a new Tab on Firefox and type: about:config;
– agree to the warning message;
– search all the lines with the string extensions.checkCompatibility and set the values to false
– restart Firefox and enable or install your “old” add-ons.
Solution 3 – The easy one!
– open a new Tab on Firefox and click on Tools –> Add-ons;
– search for the Add-on Compatibility Reporter and install it;
– restart Firefox and you will be able to install all the old add-ons.