What data isn't getting cleared for you?

I hadn't tested this feature before, so I set it to remove form data and cookies. I then visited a site that shows the cookies being sent. I used menu > Quit, then started Firefox again and used history to go back to that site and -- as expected, no cookies were sent.

So I think it should work, but I didn't want to remove my history, so it wasn't a complete test.

Do you use any of the Android Developer options? There is one called "Don't keep activities" that can interact with Firefox in strange ways.

Actually history is what is not being cleared. I must shamefully admit I did not check if any of the other items were being cleared. After you mentioned it, I did some searching and it appears to be surprising difficult to find tools to view the various pieces of privacy data. Suggestions?

The Developer Menu is enabled but "Don't keep activities" is not enabled.

I need to add a couple of details: every so often it works as expected; adblock is installed and has been for some time. I would be a bit surprised if that was the cause but I've been surprised before.

Perhaps I should just do an uninstall/reinstall of Firefox and adblock...

Doing unistall/reinstall did the trick and all is working as it should.