My Firefox has just updated itself to 1.5 RC1. Seems to be pretty new - so far, neither Mozillazine not Slashdot have mentioned it yet. This may have changed when you read this, though.
From the release notes, it’s just bug fixes. Which are always a good idea. 
Auto-update should work - to me, it did. If not, you can go to the Help menu and select “Check for Updates”. If you’re really desperate… nah, you shouldn’t be. Testing the auto-update system is good.
Note that the About box says it’s version “1.5″, but it’s not - it’s a release candidate. Then again, from my experience the 1.5 pre-release versions are already more stable than 1.0.7, not to mention faster, with a better Preferences interface, and the ability to re-order tabs by dragging them.
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I have just tried it out, and what I observed is that there may be some problems with some new features. I suggest you check out MozillaZine - on my blog.