Rated 5 out of 5 stars

le By_Myself ar Feabh. 19, 2009 · buan-nasc

To make it "work" in firefox 3.x, just open the .xpi file in your zip program, then open and edit(In note/wordpad) install.rdf.
Then look for:

and

just change 2 to 4 or your current firefox version.

But if you are as lasy as me, then just get it here:
http://randomcrap.dk/temp/minimizetotray-0.0.1.2006102615+-fx+zm+sb+tb_mod.xpi
All i changed was maxVersion to 4.
But I cant guarantee it works for you.