I've just installed the latest beta of Firefox because my friend daniel said it rocked on the mac and that he was thinking about not using Safari anymore.
The firts launch is amazingly fast, the windows just pop up, that is very impressive. I like that a lot. It also seems that Firefox is way more responsive on my G5 iMac than the previous 2.0 series. I like the new cocoa dropdown list and the look and feel - looks a lot like Camino :-). I tend to access a few sites with certificates+ssl authentification, and one of the server I go to daily as an expired cert. In 2.0 a alert would pop up and I would just hit enter to access the site. In 3.0b it'a a complete different story, the page does not load and Firefox quives you kind of a 404 page on which you can decide to make an execption or not. I think those "security" changes are very good because they really force the user to acknowledge that there is a security issue.
Things I don't like is firts the fact that non of the extensions I use daily (spell checking ), del.icio.us and a few others are not compatible with the Beta releases. Making the whole experience of testing the beta release a less enjoyable experience. I don't like the new url bar history - it's too big makes it difficult for me to use the down arrow to select the site i want to use. And the biggest bad point is I was unable to access the new Yahoo! mail, I either need to use another browser to access it or to return to the old interface of yahoo mail.