[meta] Opening links outside of the sandbox

[kmk3]

There are multiple discussions about opening links from inside of the sandbox
in a program outside of the sandbox (or in a separate sandbox).

Unlike issues, links are not automatically cross-referenced in GitHub
discussions, so create this meta issue to keep track of all of them.

Relates to:

• JDownloader: cannot execute web browser #2336
• Properly document how to "Drastically reduce security for profiles which need to open stuff in [browser name]" #3471
• JDownloader: cannot open links in firejailed Firefox #3579
• thunderbird: cannot open links in Firefox: "Your Firefox profile cannot be loaded" #3759
• Allowing calling specific apps outside the sandbox or with a different firejail profile  #3785
• xdg-open inside a jail can't open URLs in (not jailed) Firefox #3919
• thunderbird: cannot open links in firefox (and vice-versa) #3982
• Controlled launch of another sandbox from inside of a sandbox? #4557
• thunderbird: cannot open email links in firefox #4658
• profiles: telegram: allow opening links (xdg-open) #4783
• telegram: cannot open links in browser #4784
• Opening URLs outside the sandbox using unix sockets #5160
• The firejail URL open game: Level fireurl unlocked #5364
• An exec directive #5406
• Allow opening hyperlinks #5566
• feature: add fireurl #5574
• Custom URL handlers and fireurl #5582
• Is there a program that listens to URL requests on a unix socket and opens appropriate programs? #5688
• Symlink handling woes #5980
• akregator: cannot open links in Firefox #6519

Current workarounds:

• https://github.com/rusty-snake/fireurl
• Custom URL handlers and fireurl #5582