The firefox vs iceweasel thing goes on and on with no end in sight. When all is said and done, it's really down to personal preference. All I can say is that I've run iceweasel since the v3.0.x in lenny and not had any serious issues. Nowadays I run v8, after going through v3.5.x, 3.6.x, 4, 5, 6 and 7, from the mozilla.debian.net repo and have never had a single problem with any of them. I cannot tell the difference between it and the firefox I use at work apart from the branding...
If you must use firefox though and you want it to keep itself up to date, then install it in your home directory somewhere and create a launcher. you will also need symlinks to plugins, etc. IMO it's just a lot easier to use iceweasel.