Firefox can be made to drop this compatibility check. To be able to install an addon which is incompatible, the compatibility check needs to be turned off.
Go to about:config and right-click anywhere and choose New > Boolean. Give the preference the following name :
For Nightly and UX-builds : extensions.checkCompatibility.nightly and set it to false.
For Aurora : extensions.checkCompatibility.7.0a and set it to false.
Older addons can now be installed on this Firefox.
For instance I went to : https://addons.mozilla.org/en-US/firefox/addon/greasemonkey/ , the page says : Not available for Firefox 8.0a1. Just right-click the Add to Firefox button and choose Open Link in New Tab.. The addon will now be installed.
Do note if they are not really compatible, Firefox will crash!
Link : http://kb.mozillazine.org/Extensions.checkCompatibility