https://support.mozilla.com/en-US/kb/Firefox%20crashes#w_get-help-fixing-this-crash

Type about:crashes in the URL bar and hit Enter.

1. Middle-Click the hyperlinks to bring up each Crash Report page in a new Tab; each page may take a few minutes to load.
2. Then copy'n'paste the URL of each of those reports into the message box here.
3. We'll see if we can help you figure out what is causing those crashes to happen.

Many previous releases ARE accessible on the Mozilla.org site:

(1) Go to "Mozilla.org" (all go-to's and clicks, do not include the quotation marks, they just delineate where you go to or what you click on!).

(2) Click on "directory" (in the "community map" column at the bottom of the page).

(3) Click on "releases.mozilla.org/".

(4) Click on "pub/".

(5) Click on "mozilla.org/".

(6) Click on "firefox/".

(7) Click on "releases/". SHORTCUT: go to "releases.mozilla.org/pub/mozilla.org/firefox/releases/".

(8) Click on whichever version of Firefox you want to roll back to--in your case, probably "3.6.24/".

(9) Click on "win32".

(10) Click on "en-US/" Or, in your case, you can more simply go directly to "releases.mozilla.org/pub/mozilla.org/firefox/releases/3.6.24/win32/en-US".

(11) Click on "Firefox Setup 3.6.24.exe".

(12) In the download box, click "save file".

(13) Close Firefox (and preferably all programs), go to wherever your downloaded files are saved, find your new Firefox file and double-click it to start the installation process.