If you really think this is a Zimbra problem then I suggest you file an entry in
bugzilla with as much supporting evidence as you can gather. FWIW, I don't see this behaviour on FF2. I have three copies of FF open with 90 tabs in total (inc 4 zimbra web clients) and 10 add-ons installed, I usually leave the browser open permanently and don't have any problems with it. The memory consumption is about 300MB and stays around there, I have also made the 'memory tuning' changes listed in many of the articles about FF and it's memory problems.