Instructions for changing version settings Rated 5 out of 5 stars

For people who do not want to change the settings of the add-on themselves as described below by some other reviewers: Nicorac has published on April 16 an already adapted version of Aardvark that works with Firefox 4. He provided a link in his posting. He says that he has only changed the version info as described by many people in their reviews. You cannot install from the .xpi file directly, you have to open the file from Firefox 4 (Firefox button > New tab > Open file).
If you to want to absolutely follow the safest route, because the adapted version has not been approved by Mozilla, you have to make the small change yourself. I have found the instructions for changing the version info in some of the reviews below a little too short and looked elsewhere. I discovered that more detailed instructions can be found at: step 4 of those instructions change 3.6.* to 4.0.*
In order to download the xpi file instead of installing it from the Mozilla page (which will fail in Firefox 4), rightclick on the downloadlink for Aardvark and select "save target as".
Hope this helps.