They are accessible via the Help menu.

Help > Release Notes is only going to show the release notes for the version that is currently being used.

This is where the release notes are for 3 specific versions of Firefox.
http://www.mozilla.com/en-US/firefox/3.6.13/releasenotes/
http://www.mozilla.com/en-US/firefox/3.6.12/releasenotes/
http://www.mozilla.com/en-US/firefox/3.5.16/releasenotes/
I can't find an index page where a user can click on the various versions and see the release notes for a specific version bef0re installing a specific version.

You could use this as a template and insert the version number you are interested in reading the release notes for.
http://www.mozilla.com/en-US/firefox/-----/releasenotes/
Save it as a bookmark and when the page loads and says OOPS! We hate to say it, but we couldn’t find that page or file you’re looking for. Edit out the ----- and insert the version number you're interested in seeing the Release Notes for. Works for me going back to the Firefox 2.0 version release notes - didn't check any further back than that.

Yes, that's a bass ackwards way of finding specific version Release Notes, but without an index page that's the best way I can think of right now. That's almost as bad as Mozilla completely removing the Help files from Firefox - all "help" is online, which doesn't do a user any good if his problem is not being able to get online.

(I agree with the-edmeister; our posts crossed.)

To clarify: I "know better" than to install a new release without knowing what's changed, BEFORE I begin the upgrade process. (One person's "new feature" ... such as, hmm, putting Help online-only ... is another's "BUG!!")

This is even more apparent for those of us who routinely use the principle of least privilege on Windows: routine work is done with a non-Admin account. To install a new Firefox release, I need to switch to an Admin account. I should be able to make an informed judgment about when to do that, based on reviewing the release notes in advance... so I should be able to find the release notes, before downloading a new release, from the web site, using my non-Admin Windows account.

The simple solution is to un-break http://www.mozilla.com/en-US/firefox to once-again include a link to release notes that can be read BEFORE downloading or launching an update.

You can of course create a keyword bookmark to fill the version parameter:

• http://www.mozilla.com/en-US/firefox/%S/releasenotes/

There used to be this page, but it hasn't been maintained for for the latest releases.

• http://www.mozilla.com/en-US/firefox/releases/