When you upgrade Firefox you may end up with one or more of the extensions you are using being reported as not compatible with the new version of Firefox that you just installed. Often the cause of this isn't that the extension is really incompatible with the new version but rather that the person who created the script hasn't yet had time to update the end version number in their extension so that Firefox knows that it is compatible.
The simplest solution to this proble is to update the version number yourself in the hope that the version number is the only thing that is compatible in the extension. If doing so does stuff up the browser then you can simply change the version number back and wait for the author to provide a proper compatible version.
The first thing you need to do is to locate where Firefox has installed the extension. It can be found by going into the following series of folders. First Documents and Settings then your login name, then Application Data then Mozilla then Firefox then Profiles. There is usually only one folder there so go into that followed by extensions.
One of the folders you now see will contain the code for the extension you are looking for - there will be one folder per extension. You may need to search to determine which folder contains the extension you want. In each open the install.rdf file in notepad and look for a description near the top of the content.
For example let's say I am looking for the "Rewind/Fastforward Buttons" extension. Within the first <RDF:Description> tag the em:name attribute is set to "Rewind/Fastforward Buttons" in the particular file I am looking for.
There's another <RDF:Description> tag at the bottom of the file where you will find a em:maxVersion attribute that specifies a number less than the version of Firefox you just installed. For example it may say 3.5.* when you just installed Firefox 3.6. The first thing you need to do to resolve the version incompatibility is to change this number to a value greater than the current Firefox version you just installed (using * to represent any value in that part of the number). Save this change to the file.
That by itself will only work if you make the change before you run the Firefox update. If you are fixing it afterwards then you need to reinstall the modified extension. To do this select all the files and folders that belong to the extension (everything in the folder where you found the install.rdf file). You then need to create a single zip file that contains all these files and folders and save it somewhere temporarily with a .xpi extension. For example To update the rewind/fastforward extension I created a zip file called ff.xpi and saved that temporarily to my desktop. The final step to reinstall the extensiopn is to open Firefox and then drag your xpi file into the browser. Firefox will then reinstall the extension which now has a max version number high enough that Firefox believes it to be compatible with the current browser version.
Provided that the maxversion field is the only incompatibility with the browser version you have now updated the extension yourself to make it compatible with the new version of Firefox.
This article written by Stephen Chapman, Felgall Pty Ltd.