Quoting Andrew McGlashan (andrew.mcglashan@affinityvision.com.au):
Love them both. (Reminder, absent a compelling reason otherwise, it's best to get these and other bits of system software via maintained distro packages, not from upstream.)
https://addons.mozilla.org/en-US/firefox/addon/change-referer-button/
Users should be aware that this is proprietary software. (One of the flaws of the addons.mozilla.org site is that it is careless and not very informative about software licensing. In general, if the obscure note in the bottom right says 'Released under Custom License', that will (90+% of the time) turn out to be euphemism for proprietary.) In this case, the developer, who goes by the handle 'cK-LFC' and does not provide his/her name and contact data, omitted any specific licence statement, which has the perhaps-intended-but-probably-not effect of proprietary terms by default. Which in turn means that nobody but the author has the legal right to maintain or even redistribute it. If you have any way to contact the author, you might suggest he elect something obvious like MPL 1.1.
Interesting. It's under GPLv3.
https://addons.mozilla.org/en-US/firefox/addon/toggle-cookies/
Open source under (somewhat obsolete) licence MPL 2.0. This is one of the exceptions to the general rule that addons.mozilla.org usually means proprietary when it says 'Released under Custom License'.