I've heard other people say this too, but it must be a workflow or use case thing. I run Firefox as my browser of choice on Linux, Mac, and Windows every day, and have for 10 years at least. I do remember a time when memory was an issue, but those problems have been resolved for a few years. I routinely have dozens of tabs open, often with Flash video or lots of javascript, sometimes 50 or more (with the excellent TreeStyleTabs plugin for hierarchical side tabs) and I have never had these problems. I restart my browser maybe once a month, usually along with OS updates.