Yeah, you can download the xpi locally and change the extension to .zip
Open that like you would with a normal zip archive. So now you have a folder. Inside that folder find the file called install.rdf and open that in a text editor. Don’t use Word, use something like Notepad.
In that file you will find the line:
<em:maxVersion>2.0.0.*</em:maxVersion>
Change that to:
<em:maxVersion>3.0.0.*</em:maxVersion>
Save that and zip that back up. Change the zipped files extension back to a .xpi
Then you can double click that and firefox should install it.
Keep in mind that doing this is not recommended and could break stuff.