Have you upgraded to the lastest version (1.07), it has stopped popups very efficently on OSX and Windows XP from my experiences.
I am at 1.07 I guess they are the "pop-unders" as described previously.
I've been using Firefox for a while now - I like it. I like the minimalist nature of the browser. (I don't install extensions)
There's a beta of the upcoming v1.5 (current beta is called Release Candidate 3) you can download at the Mozilla site, the 1.5 version includes auto-notify of program upgrades and says it will update online.
Presumably that means users will no longer need to uninstall then re-install to upgrade. I received an auto-notify of the upgrade from rc2 to rc3 a week or two ago, but upgraded manually the old way. I might try the upgrade option next time.
Don't get popups. Ever. Not sure which layer of security is responsible for that though.
Currently posting this on 1.5rc3 - curious to see whether I'll have an opportunity to upgrade to the final version via online update next week. :)