I recently switched from the stable Firefox 3.6x browser to the beta of Firefox 4, mainly because the beta appeared to be relatively stable at this point in development. Naturally, some of the add-ons that I used to work with were not compatible with Firefox 4, either because the developer did not update the add-on yet or because the development has been abandoned and the extension won't receive any new updates.
One of the add-ons that I came to rely on in the past is the Greasemonkey extension which allows you to load userscripts into the browser. Unfortunately it was one of the add-ons that is not working in Firefox 4 Beta.
I first tried to force compatibility on the add-on, with mixed results as it was not working correctly afterwards (take a look at this guide on how to force compatibility in the Firefox web browser)
If you look at the official Greasemonkey site on Mozilla you notice that the add-on is only compatible with versions up to Firefox 3.6, and the same information is offered on the web page of the developer. The last release dates back to April 2010.
What many users do not know is that the developer publishes nightly builds of Greasemonkey on the same server. Nightly builds are development versions of the add-on which often contain additional features or improvements.
In the case of Greasemonkey, they add Firefox 4 compatibility. Just install the latest nightly build from the build list to make Greasemonkey compatible with Firefox 4. Please note though that nightly builds may contain bugs. Firefox will automatically recognize Greasemonkey nightly updates and offer to install them automatically, so that it does not need to be done manually.
All installed userscripts worked fine after installation of the latest Greasemonkey nightly. Let us know your experience in the comments.
Update: Firefox 4 final has been released in the meantime and Greasemonkey too was released in a version compatible with the new version of Firefox.
The information presented here may still be useful to Firefox users if a similar situation happens again in the future. If Greasemonkey becomes incompatible, check out the nightly builds of the browser add-on to see if an update is in the works that will make it compatible again with the latest version of Firefox.