LONG ONE, PLEASE READ, I think I have good idea!

Ok, this has been one of my favorite add-ons for way over a decade, IF I recall at all how long this one has been around. And it's certainly not a new one! What I'd like to have is paste-services - it's (main) purpose wouldn't be sending the whole page content (in text or html), though a pop-up asking, wouldn't be bad... However, it should upload the currently selected area - an image, or piece of text: depending on what the particular paste/upload service is meant for. Also maybe at least, when user has selected nothing, maybe even if he did, and it's a text-paste service (like most are), there should be a popup with maybe two option list (choose-one) with "upload the content" or "upload the HTML of [page|area]". Now, it's unlikenly that if area is selected (unless if in inspector/source-view - adding support for inspector of course isn't really this add-ons maybe purpose and if in source-view mode, it won't have to ask anyway. And if user selected an area already, he likely want's what hun sees, no matter what view hun is in.

If nothing was selected, maybe a warning should be given in a pop-up and option to choose "Cancel" - "Paste text-content of the page" (formatting it in plain text, as well as you could - firefox *might* have functionality to provide such rendering as well - after all, it has the "Reader mode" built-in.
Or, if image upload service, it could suggest "Cancel", "Full-page screenshot", "Visible area screenshot", or additionally even "Select area from page". It just popped to my mind when I had to paste content to one of these services, Mozilla's dpaste (https://paste.mozilla.org/) and instantly I though, that's the kinda stuff FlagFox might have; though later I realized I confused it with short-URL services. Pastebin and such would be a nice addition, even if they don't work exactly like the rest of you stuff.

I understand if that's not direction you're want to go, but IMHO it would fit your add-on's functionality, extending it's purpose, not adding something totally different :)

Love you extension, no matter what. And thanks for not abandoning FF when Legacy support went away - too many of the greatest didn't stay :( Some still developed for forks of FF with Legacy support added back - but I haven't gone to them now; but I digress.
Yours is such a great example of rather simple add-on, it basically just offers redirection to pages that offer something to do with the URL, but it's so valuable. One tool for one thing, which is what it does in excellent manner. That's the UNIX principle, though it's functionality was based on combining these tools to create endless possibilities on how they can be used, but still: As extensions go, there's other reasons why they often work best. Not to say some of the best add-ons I've used have also been AIO add-ons. But for most time, add-ons are best to do one thing and do that one good perfectly <3

Good luck, and hope you consider - there's no way to invent this as adding custom action in proper way I meant - ok, I'll digress again, but had this funny idea (about adding the services myself):
"unless I built a website that would run an actual FireFox in headless X-server, communicating by it's own extension I'd write with the server-side tool, and that "middle-site" would decide and ask those questions... But the result wouldn't be from your browser, it would be clumsy
and something that 5-10 years ago, I would have written for fun of it, to get this functionality for myself :DDDD
And I could have just written my own add-on for that, but I love hacking a way to use the existing tool and my own tools to get the original add-on achieve more than it has been given functionality. It's about having fun.
Of course, making an unofficial fork of yours and adding the feature, would make much more sense - though, not sure about your license would allow me to spread my mod - as more than a patch, that users could use to add my changes, at their own risk though... Is it FOSS, because that right there would gain you the last star! Not because I want to fork it, I don't, I just highly appreciate people who use FOSS license (I hope also for it's benefits, it's not a "charity" licence, you'll also gain community willing to support with... anything, not just code path suggestions or code, but discussion about features, hosting, anything...). But it's not the only way of getting five stars, heck most 5 star reviews I've given were not for FOSS projects at all. But this feature, which I believe fits to your tools purpose, it would extend it with something similar with slight difference.