99% Firefox
I've been using it for more than a year, under Win98SE and WinXP, with very few problems.
They just released a new benchmark (1.5), and it includes a 'context menu' item (right click) to report sites that don't work in Firefox... I guess, so Mozilla can send them a message.
If you decide to go with it, and you run into snags, post them here. Either we've run into it before and have a fix, or we can team up on it.