A large part of the problem here is that this isn't a simple 1->2->3 progression. There are days when I can use firefox all day and have it be snappy and only use about 100MB active with about 500MB in virtual memory.
There are other days, however, when Firefox uses 700MB of active memory and nearly 2.5GB of virtual space. Now, this is the result of 2-5 days worth of browsing. How in your right mind do you expect me to keep track of every click that I made over 5 days so that you can file a bug report? Do you have a keytrapper and site visitation storage engine that keeps up every image, flash video and html of the site? After all, it could be any of that. I don't mind trying to help out, and I'm sure a large portion of the FF community doesn't either, but we've gone beyond what the end users can do to help you with this issue, percieved or real, and it's now in your hands to be the developer and QA this to find out what is going on.