I don't use the browsing protection extension, but neither Firefox nor SAFE give the option to just remove it.
Sorry for my reply. I'm also only F-Secure user (their Home Solutions);
I able to think that 'designed situation' covered by this article:
Extensions installed by other software
Sometimes installing or updating other software will add an extension which can't be removed from within the Firefox Add-ons manager because its Remove button is not functional.
The software that added the Firefox extension may include an option to remove it. You should visit the support site of the software provider for more information. As an alternative, simply disable the extension in the Firefox Add-ons manager if you no longer wish to use it. To disable the extension, select it, then click on its Disable button. The extension will be disabled after you restart Firefox.
And potential hack-trick with manual removing related '.xpi-file' (most likely work, but not sure what if F-Secure will try to reinstall/redownload it);
But I able to think that "disable"-state can be enough (or if not -> maybe good to ask as "feature request" for ability to "Remove" it by common steps);
Browsing Protection extension about "HTTPS-support helper" (rating picture under HTTPS search engines Google/Yahoo/Bing; Some tweaks for detect and trigger Banking Protection session; maybe some other things);