Any thoughts about why Firefox is frozen? Does it appear to be waiting for some kind of input, or is it just "not responding" at this point?

Please try backing up your current session tabs as follows -- this assumes you are save history and are not using private browsing:

Type or paste the following into the Windows Run dialog or the Windows 7 start menu search box and press Enter to launch Windows Explorer:

%APPDATA%\Mozilla\Firefox\Profiles

In that folder, there may be a single profile folder which a name like a1b2c3d4.default (or some such half-random name). Double-click into that folder and then into the sessionstore-backups folder.

Try to right-click > Copy each of the following files and right-click > Paset them somewhere like your desktop:

• recovery.jsonlz4: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session)
• recovery.baklz4: a backup copy of recovery.jsonlz4
• previous.jsonlz4: the windows and tabs in your last Firefox session

Hopefully the first two are up-to-date as of the time Firefox became nonresponsive, and the third could be useful in case there is a problem with the first two.

After that, you could "kill" Firefox in the Windows 7 Task Manager. There's a reasonable chance that crash recovery will kick in but, if not, and if "Restore Previous Session" doesn't work, we can assist in using those files you backed up.

Can you open a new window?