I get along just fine without closing Firefox or restarting my laptop for like a month and having 10-20 windows (windows, not tabs, I hate tabs) open at any one time. Sometime Firefox does crash if you haven't restarted it or the computer for a while, but sometimes does mean sometimes - once a month of use as outlined above.

An important question though: what kind of lag is it?

Is it internet lag, such as web sites taking a long time to load, or links needing to be clicked a few times before they actually work, or just bad responsiveness in general..

OR - is the Firefox itself lagging? Such as slow opening of new windows/tabs, slow minimizing or restoring of windows, slow scrolling on a page that's already loaded?

The first one is internet related. The second one is RAM or bad add-on related. An overly protective antivirus could cause both, the first one by being overly protective of web traffic, and the second one indirectly, by eating up all of available resources on older computers.