I tend to have lots of browser tabs open. Even more so with Firefox 4’s Panorama feature, which I find handy despite the frustrating limitation that tab groups cannot be moved between windows. Alas, having many tabs open is suspected to trigger/exacerbate a Firefox memory leak when Adblock Plus is running. My own informal testing corroborates this.
In Firefox 3.6 I had used BarTab, a great plug-in that allowed unloading tabs without closing them to reclaim memory. Alas, it is not yet compatible with Firefox 4.
But it turns out Firefox 4 has a new setting (called Cascaded Session Restore) that provides a reasonable work-around. In about:config set
browser.sessionstore.max_concurrent_tabs=0
…and then, when closed and re-opened, Firefox will not load any tab until you click on it. Now it’s extremely fast to quit, restart, and continue on with reduced memory usage.