It is supposed to do exactly what I needed on Firefox for Android. But the background tab is not automatically opened why switching to it, and the Reload button does not work either, I have to tap on the address bar and type Enter, so I cannot say that it really works, but it is an excellent try.

Another remark: the option window is accessible through the plugin manager, but not directly by clicking on the plugin name in the plugin list, which works with other plugins. And the title "blacklist" is not obvious, it could be "apply only to URL matching on of these patterns" or "do not apply to URL matching...".

Edit after your answer: it does works in some cases, in contrast to my first tests. According to my tests, it works even when Firefox is killed and its memory released. But sometimes not: I have the feeling that it fails when Firefox itself decides to discard a tab, maybe because there are too much of them or because if was not used for some time. Firefox for Android may not have the "discard" function, it obviously has some internal way to keep a tab discarded and to refresh it when selected. But possibly there is no way to do it from Javascript in an addon. I am afraid I cannot help.

Sorry, I am not adventurous enough to play with "modern" Javascript (too abstract: for me, bare-metal C and VHDL already are high-level languages, and 10 kB is a big executable).