Well, generally speaking, how does this differ from checking any other software if the development of said software was not dropped? You usually do not have anything like this. You can be only notified about SW updates. I understand your request, but it is somewhat impossible to achieve, in my opinion.

What I get from you is that you want an addon which would send a notification to all of its users when the project gets dropped. Consequently, someone would have to write and send that notification manually, meaning someone would have to check regularly whether the project is still active and up-to-date. You still rely on that someone to do their work and not abandon this addon as well. If developer team announcements are not good enough, nothing and no one can be possibly trustworthy enough to guarantee the notification will be sent when it is due. There will always have to be someone who needs to manage all this and you would have to trust this someone to do their job. What would be the metrics to consider the project abandoned, anyway? That someone would have to somehow deduce that a certain project is abandoned, if we are speaking about SW in general.

Back to LW and Firefox specifically, If what you want is to compare the versions of LW with Firefox each time LW starts (or periodically, for example) and this is all you are worried about, why not. Be sure you do not send false notifications every time Firefox gets updated and LW team has a few days before LW gets the update. If the time between Firefox and LW updates is the metrics by which you consider a project being abandoned, that is. But I do not believe this would work in practise. You rely on LW having the same structure versioning as Firefox in the future, no changes from the upstream release cycle etc. What if Mozilla stops updating Firefox, for example? Then the upstream project would be abandoned but the addon would still happily report everything is up-to-date. It is possible to write such a script, I am just a bit sceptical about its usefulness.

If this is the solution you require, then no, nothing like this exists as far as I know, and yes, the script to do this would work as described, but I am not ultimately sure if this is desired. And I honestly think this would cause more disinformation and discomfort for the user than it would be useful. I wish you good luck nevertheless. Perhaps I am completely wrong and it is a brilliant idea. Hope you find a satisfiable solution that would work for all of you.