If you have updated Firefox and/or Thunderbird and have found that some of your favourite add ons no longer work, there are a number of ways to make them work.
One way, that I have written about for Firefox and for Thunderbird, is to install Mr. Tech Toolkit. But, the problem with that add on is that it is outdated and incompatible and must be adjusted to be made to work. Also, currently, the forced-to-work version of Mr Tech Toolkit will also cause the Add-Ons Manager page to show in a way that makes it difficult to close it. If you want to switch from using that add on to using the Add-on Compatibility Reporter (see below), install the reporter add on and disable the toolkit add on and restart (in either Firefox or Thunderbird).
The Add-on Compatibility Reporter
This is an add-on made by Mozilla that will enable all old add-ons and will also let you send a report to Mozilla that a certain old add-on still works fine in the new version of Firefox or Thunderbird if you so desire.
Here is the version for Thunderbird.
There is an issue with forcing old add ons to work. Sometimes they don't work and can cause problems. The only way to troubleshoot this is to disable them one by one and restart to test things out. Sometimes an add-on can cause a very odd specific problem. Once you find out which add on is causing the problem, disable it and keep it disabled. If you don't need it anymore, uninstall it. But, if you would like to use it if it would work, keep it disabled but check once in a while to see if it is updated. If it is ever properly updated, it will usually stop being labelled as incompatible with the current version on the Add Ons Manager list. Once you see this you could give it a try again - enable it - and see if it still causes the old problem.
Here are a couple of articles from ghacks.net discussing the Add-on Compatibility Reporter and other methods of forcing compatibility and the issues around this.
Firefox Add-on Compatibility Reporter, Force Add-On Compatibility
Three Methods To Force Add-On Compatibility in Firefox
One way, that I have written about for Firefox and for Thunderbird, is to install Mr. Tech Toolkit. But, the problem with that add on is that it is outdated and incompatible and must be adjusted to be made to work. Also, currently, the forced-to-work version of Mr Tech Toolkit will also cause the Add-Ons Manager page to show in a way that makes it difficult to close it. If you want to switch from using that add on to using the Add-on Compatibility Reporter (see below), install the reporter add on and disable the toolkit add on and restart (in either Firefox or Thunderbird).
The Add-on Compatibility Reporter
This is an add-on made by Mozilla that will enable all old add-ons and will also let you send a report to Mozilla that a certain old add-on still works fine in the new version of Firefox or Thunderbird if you so desire.
Here is the version for Thunderbird.
There is an issue with forcing old add ons to work. Sometimes they don't work and can cause problems. The only way to troubleshoot this is to disable them one by one and restart to test things out. Sometimes an add-on can cause a very odd specific problem. Once you find out which add on is causing the problem, disable it and keep it disabled. If you don't need it anymore, uninstall it. But, if you would like to use it if it would work, keep it disabled but check once in a while to see if it is updated. If it is ever properly updated, it will usually stop being labelled as incompatible with the current version on the Add Ons Manager list. Once you see this you could give it a try again - enable it - and see if it still causes the old problem.
Here are a couple of articles from ghacks.net discussing the Add-on Compatibility Reporter and other methods of forcing compatibility and the issues around this.
Firefox Add-on Compatibility Reporter, Force Add-On Compatibility
Three Methods To Force Add-On Compatibility in Firefox
