"Nightly", "Beta" etc aren't stable versions of Firefox. So it'll be better if you install the stable version from playstore. Playstore link: https://play.google.com/store/apps/details?id=org.mozilla.firefox&hl=en

And the nightly version updates every nights, so it's called nightly.

Please do consider creating a bug in bugzilla.mozilla.org with the Nightly and mobile components selected. Since Nightly is updated everyday, you may be the first to report it as well as might see it fixed in the next version. Update first, then please consider filing a bug.

As a reply to my own question, I add my suspicion; maybe someone has an idea what to do.

Firstly, the stable version was showing the problem; that's why I turned to nightly. I know there might be some glitches, but it is better than nothing at all. And, the next day may have solved the glitch (because it is updated daily).

Secondly, since I am in China, the Google Playstore is not accessible (blocked), so I have to sideload every version manually. Nightly, however, downloads (albeit slow), automatically.

I suspect, the behavior is caused by lack of memory. I noticed, after a phone re-start Firefox would run better, also Chrome does not do its "Aw-Snap" so much. So, I tried to delete some files to get space, and indeed, the situation improved. Now, 130 MB of 4 GB available.

Unfortunately, I don't know how much memory is needed, and how to control that. I almost have no pictures, videos, music, etc in that store, all on the additional SD card.

I think that the new memory manager is coming out in the OS, but for now it is possible to achieve the memory management with https://play.google.com/store/apps/details?id=com.jrummy.apps.memory.ma...

Also as for how much memory is needed for Firefox for Android. The most recent one I could find is: "The binary is around 35 MB, uncompressed after install it is about 60MB" https://support.mozilla.org/en-US/que.../1027304