A very convenient trick: having a testing browser
One of the little peculiarities of my personal environment is that I actually use two different copies of Firefox at once. Well, sort of. What I have is my regular Firefox and a separate 'testing' instance.
There are two important configuration differences between the instances. First, the testing instance uses a standard version of Firefox, instead of my usual self-compiled and custom-hacked version. Second, it is configured to clear all 'history' when Firefox exits, thereby flushing browsing history, the cache, cookies, and so on.
(Somewhat paradoxically, I also use the testing Firefox for high security things. The great session isolation cuts both ways, making my high security browsing isolated from everything else provided that I'm only using the testing Firefox instance for the high security browsing at the time.)
Since the testing Firefox only cleans itself up when it shuts down, I'm usually not running it and when I do, I shut it down frequently. In stark contrast I keep my regular browsing environment running all the time so I can use Firefox's remote control to start new windows rapidly, and I often have lots of iconified Firefox windows lying around.
Since the testing Firefox environment is ultimately disposable, it's also made a convenient place to test different versions of Firefox. If I want to do semi-serious browsing with a Firefox 4 beta or the like, I point it at the testing environment and see how things go.
(For reasons beyond the scope of this entry, my testing instance currently runs the latest mozilla.org 32-bit Firefox build instead of the system version of Firefox. Since I got Flash working again in the system Firefox, I should probably revert that. Mind you, there are arguments either way for testing purposes, given that very few of our users are likely to be running on 64-bit Fedora Linux while a lot more are probably using some version of mozilla.org's official build.)
You don't have to do this with a second instance of Firefox, of course. The important thing is to configure whatever you use as your testing browser to clear everything when it shuts down, so that you always start browsing from a known (and clean) state.
PS: given the parade of security issues in Firefox I strongly recommend that everyone uses NoScript in a 'default disabled' mode in at least their main browsing session, and I normally use NoScript this way even in my testing Firefox. It is much less annoying than you might think, especially if you're willing to whitelist things more liberally than I am.
(Yes, I have been beating this particular drum for some time.)