Another way to Rated 5 out of 5 stars
I downloaded the file from the link Karl posted & then followed what this link told me to do & I opened/viewed the archive within a program like Winzip (not unzipped it) & viewed it within wordpad & changed the maxVersion number listed under Firefox to what you want (ex: maxVersion="19.0a1) then I saved it... instead of saving it as a .txt save it as install.rdf on your desktop then drag & drop the new file from your desktop into the old xpi file that you are viewing within winzip & it should replace the old file with the new one.. then just drag the xpi file into the open browser & tada .. it's quite easy if you follow the instructions!!! It worked for me though I still wish this addon would be updated for later versions..
This option I noticed doesn't give me the warning that addon compatibly check is disabled..
NOTE in the example... I put maxVersion="19.0a1 incase Mozilla updates to 19 in the future.. so I don't have to constantly update the XPI file :)