This is not true. The problem lies not in Firefox per se, but in the authentication mechanisms used to access the site. When you flash your router to DD-WRT, you will be prompted to change the username and password. Firefox (but apparently not IE) doesn’t like it when a site suddenly switches authentication regimes after having previously authenticated. (IE apparently handles the situation better. I don’t know the details since I have a Mac and I haven’t tried it with IE.)

The workaround is simple: in Firefox, just go to the Tools menu and click “Clear recent history…” Wipe out any active logins and cookies in the last hour (or however long it’s been since you started messing with DD-WRT), and it should work fine. (If you’re using the Web Developer plugin, you can do the same thing under “Miscellaneous -> Clear Private Data”.) Hit “refresh” on a page belonging to your DD-WRT router, and you should be prompted to log in again. After that, everything should work fine.