Right here right now
If you want to make sure you have the last version of Firefox, try this site (using javascript and SVG) or this one where you will play Tetris.
[update] The MD5 for 1.5 is exactly the same as for 1.5 RC3 (d0cbbd5d8c47fe36ee8f26fb1255838c).
It simply means that Firefox 1.5 RC3 = Firefox 1.5.
So, if you’re using Firefox 1.5 Release Candidate 3, congratulations: you have Firefox 1.5.

[update2] 1337tech.org did a great job in measuring F1.5 perfomances.