Why do you keep the tabs open after you've answered their question?
So, I have a lot of reference materials open on my desk, and I keep them open because finding the page I was on takes a long time, and loading that page takes even longer.
Some people might not know that it actually includes a feature specifically for this use case. If you start to type in a URL or page name similar to one already open in a tab, the awesomebar dropdown menu lets you jump to the already-opened tab.
And I'm not sure I really like it anyway - when I re-type a URL I'm usually looking to reload it to grab a fresh copy. I tab-complete the address of my development server and Hacker News very often; anything else I don't usually know the URL off the top of my head, I just remember that I have such and such doc on such and such desktop.
eg: % Firefox 7 memory
Meantime being careful about what I close does a decent job, and I don't have to maintain a proxy server.
Finally I have two blank desktops that tend to get eaten up by tabs that I use when people come into my office asking questions that I don't know the precise answer to off the top of my head.
But yes, on a single desktop about 12 tabs is my limit. And when I get over 4 full browser windows I start to get antsy and know that I can't possibly keep focused on everything I'm doing.