I'm not surprised it takes a long time to clear history. How long have you let it run before giving up and using force quit?

Apart from that the fact that clearing a large amount of history can take a while, when you force quit Firefox, you run the risk of corrupting the data in your profile folder.

To get rid of all your history,

1. Create a manual backup of your bookmarks.
2. Open your profile folder.
3. Quit Firefox.
4. Rename the places.sqlite file in your profile folder to places.sqlite-old
5. Restore the bookmarks you backed up earlier (detailed directions are in the same article in the first step).
6. If everything went well, delete the places.sqlite-old file.

You can check for problems with the places.sqlite database file in the Firefox profile folder.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.org/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file

I followed your instructions Gingerbread Man & I think it worked...the only thing different was my bookmarks weren't removed, so I didn't have to restore them. I cleared my browsing history as a test afterward & it worked fine. Thank you!