Firefox is on a rapid release schedule since version 4.0. This makes it harder for a lot of addons to catch up with the newer releases. By default, Firefox refuses to install any addon that is not compatible with its current version.
Firefox can be made to drop this compatibility check. Consider an addon that works with Firefox version 4.0, but has not been updated for version 5.0. To be able to install this addon on Firefox 5.0, the compatibility check for 5.0 needs to be turned off.
As an example, consider turning off compatibility check in Firefox 5.0. Go to
about:config and right-click anywhere and choose New → Boolean. Give the preference name as
extensions.checkCompatibility.5.0 and set it to
false. Here, replace
5.0 with your Firefox version number. Older addons can now be installed on this Firefox. Do note if they are not really compatible, Firefox will crash! 😊
For more read extensions.checkCompatibility
Tried with: Firefox 5.0