As expressed in a previous post I'm quite happy about the improvements of Firefox 4 even at the expense of not being able to run a few add-on's which have not yet been upgraded to run in this version. After a bit of research, however, I found out that what keeps add-on's from installing in FF 4 is just the highest supported version information in the add-on. However, that can be changed easily. And here's how:
Download the add-on to the desktop with a right-click and then "save as" instead of trying to install it. You should then have an .xpi file. Rename it to .zip, because that's basically what it is and open it. You'll find a directory structure in which root there are two files, one of them being called "install.rdf". Open it with a text editor and modify the following line to say "4.0":
Then save the file and make sure it ends up in the .zip file again. Next, rename .zip back to .xpi and open the file in Firefox. This will start the add-on installer. And that's it. Of course there is no guarantee the add-on will work flawlessly as there is always a chance it uses features that were changed or removed between versions. But at least for the add-on I tried this was not the case and things work flawlessly.