why? Id wipe out the previous version if its in $HOME, then just install the new one, if you leave the .mozilla or .firefox file intact you wont lose your bookmarks and preferences
this is exactly like unzipping a file onto your desktop in windows, clicking on the new directory, then clicking an .exe file..............its the exact same deal
firefox for linux unless you build it yourself is a PRECOMPILED binary, all that it needs to run is unzipped, then run its installer which all that does is check for dependencies and unpack itself, thats it