Next Firefox version: how to *easily* check extension/add-on compatibility?

Maybe the Firefox add-on Is it compatible? will help you (it helped me). It simply adds compatibility information into the list of installed add-ons.

Or perhaps you could check out the Add-on Compatibility Center.

There is a report there that lists the add-ons that make up 95% of all installed add-ons sorted by popularity. (I'd link it but there's no direct URL.)

First a remark: I suggest in any case to wait for FF 4.1, because version 4.0 is quite unstable. It was aborting frequently, so finally I just went back to FF3. I believe that the new Javascript engine still has problems.

Now to address your question : There is no sure way to know if an add-on is compatible with FF4. Even if it is marked as incompatible and disabled by FF4, it may still in actual fact be perfectly compatible. You just have to try and see, and the simplest way is to turn off the check.

To Force Firefox Extension Compatibility, version check is turned off in about:config : Right-click on the list and choose ‘New’ (see first if it is already there), followed by ‘Boolean’. Enter the name as ‘extensions.checkCompatibility’ and the value as ‘false’. Once done, extensions will no longer check to see if they are compatible with the version of Firefox you are running. You can re-enable the check again at any time by setting it to ‘true’.

A much better way of doing this, while at the same time helping the community, is presented in this article : Test and Report Add-on Compatibility in Firefox.

One installs the Add-on Compatibility Reporter, which enables all your incompatible extensions for you to test whether they still work. If you notice that one of your add-ons doesn't seem to be working the same way it did in previous versions of the application, just open the Add-ons Manager and click Compatibility next to that add-on to send a report to Mozilla. You can of course also report that an extension is working perfectly well.

You could try using Firefox Portable Edition. That way you could check out Firefox 4 and its compatibility with your add-ons without uninstalling your current version of Firefox.