I recently installed Firefox 5.0 from the packages.linuxmint.com repository and ran into two problems instantly:1.
Doing an "apt-get install firefox" installed the standard Firefox 5 but with the program not in English. I had to install the en-us package which then removed the af language package. My default language on my OS is English and haven't switched to French or any other language.2.
Font rendering is not as smooth as Iceweasel. I don't know if it is a Cairo issue or not, but Iceweasel's font quality seems to be the way it should be for the default fonts (serif, sans-serif, mono) and is the same as Chromium/Chrome on Linux and Firefox on Windows. I ran into this exact same problem with the tar.gz file from Mozilla's page for Firefox 5 and the Linux Mint version has the same font issues. Is there a setting that I need to change in my about:config or install extra? I didn't have to edit my about:config or install anything extra for Iceweasel.
If the stock Firefox for Linux is causing the problem for Debian and is not easily fixed, should the Mint team instead grab the Iceweasel source from http://mozilla.debian.net
and rebrand the browser by offering it in packages.linuxmint.com? It could be a browser extension or system-adjustments file that has the branding and Mint features integrated on top of an Iceweasel install and most users won't know the difference.
Now that Firefox has moved into a rapid release cycle, maybe the browser extension or system-adjustments file to modify Iceweasel on the fly could benefit more in the long run and less work on Clem and the team.