The reason is simple, these extensions are created by external developers and they can only fix the code or release later versions of their extensions, after the newest browser version has been released. Also, some developers don't update their code at all.
Problem - Incompatible extensions
I recently upgraded from Firefox 3.5.7 to Firefox 3.6 and found that two of my extensions didn't work anymore, GMail Manager and Easy DragToGo.
There are two ways to deal with incompatible Firefox extensions:
- Set Firefox not to check the compatibility of the extension and ignore it altogether, or
- Modify the compatibility version of the individual add-on/extension
I chose option 2 (see second section on that page) because option 1 doesn't always work with all add-ons and since you're modifying the individual add-on with option 2, there is less risk.
In short, I basically changed the value of <em:maxversion> to 3.6.* in the install.rtf file of each add-on, re-installed the updated add-on, restarted the browser and everything worked again.
Related link
Why I use Firefox (and list of extensions I use) Read more on "Make your extensions work in Firefox 3.6"!