Firefox, while a great browser, is a bit behind the times at the moment because it’s the only major player that doesn’t have a private-browsing mode. I personally think this is an important feature to have because of the enhanced security. Some jokingly call this feature "porn mode", but where I would use it most is for online banking.
So, you’re a big Firefox fan and want to see what’s in store? Firefox 3.5 beta 4 is available now for download right here.
What’s new in 3.5?
Gecko 1.9.1 rendering platform. It supports new web technologies and has improved performance.
Private browsing mode. This is what the browser needed most to keep up with the competition.
A new JavaScript engine called TraceMonkey. This adds in better performance. Webmail users (Gmail, Hotmail, etc.) in particular should notice a notable speed improvement from this.
However..
Be sure to read the Known Issues portion of the link above. It’s important to know things like:
Occasionally Google Mail will fail to respond when creating a new filter, showing advanced search options, or clearing the spam folder. Reloading Google Mail fixes the issue (see bug 477101 and bug 478778)
If you’re a Gmail user, this is a deal-breaker until that’s fixed. Take the time to read over the other known issues as well. If there’s nothing listed that sounds too bad, give FF 3.5 a shot, you might like it.
The best advice I can give for those who want to try out the bleeding-edge version of Firefox is to backup your add-ons first. Either copy the add-on XPIs directly from your associated folder, or alternatively re-download them to a backup folder in case you need to uninstall and go back to FF 3.0.10.
To re-download add-ons manually: Launch Firefox, click Tools then Add-ons, a small window will pop open listing them all. Right-click the add-on you want to re-download and from the small menu that appears click Visit Home Page. This will take you directly to the web site for that add-on where you can download it.
The browser install/uninstall isn’t the bad part, it’s always the add-ons. Having them backed up is mandatory. After all, who uses FF without add-ons anyway?

I’ve never had a good experience with Betas. And that is why I try to stay away from them.
I tried Opera 10 and Safari 4 recently; both proved to buggy for me to handle.
I’ve been running the betas on my test computer (an older Dell laptop) for a while with no problems and a couple days ago I installed it on my Computer and my wife’s. Both running XPSP3. Had to tweak a few add-ons for version number and now all works fine. Stable and nothing buggy.
I don’t know if it is hidden somewhere but I did not see tabs running as individual processes added. Chrome and IE8’s tabs are individual processes. If one crashes it doesnt take the browser with it. Is this in FF3.5 or is FF still playing catch up?
Sorry to hear you are suffering such frequent browser crashes. Can’t remember the last time my Firefox crashed … about once a month on average – usually when I’m experimenting with something or the other. Even then, click ‘firefox’ and click ‘restore tabs from last session’ and it’s back up in 5 seconds… nothing lost.
Individual processes are coming in about a year (which I have a feeling you already know) but it’s really a non-issue for me since it already runs so fast and stable.
Firefox crashed more often than IE for me until a few days ago, when IE started crashing a lot more for some reason.
I’m up to the latest beta 3.5b99 Firefox – still stable.