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Быстрый ответ: в Firefox for Android в nightly/beta сборках есть about:config.

Firefox for Android nightly можно скачать здесь (сайт TrashBox), а Firefox for Android beta - здесь (сайт TrashBox).

По поводу about:config в Firefox for Android уже говорили в 2020 году:

You can access about:config in the Nightly build of Firefox for Android, available from the Google Play store. - Paul

In response to about:config not working in Android Firefox, I have been using the nightly version but just recently a behaviour has changed. - Ben Vost

Вот ещё сообщение 2021 года:

The Android version of Firefox Daylight (Fenix) blocks about:config on the stable/release version of the app. I’m not entirely clear on the design details behind this (partly becaseu the dev response actually makes no sense), but the claimed reason is that a lot of the elements exposed by about:config no longer function with the new engine, or function differently than they do on the desktop version or did in Fennec (why about:config would even expose or contain unusable toggles in the first place in an application that has supposedly been entirely rebuilt is an open question), so it is disabled entirely in order to prevent normie users from completely borking their installations (again, that doesn’t really make sense, since a reset to default should fix any issues caused by editing config options, and the about:config page tracks and displays which options have been changed from defaults, so that reason doesn’t match reality).

The about:config page is functional in the beta and nightly releases of Firefox (Daylight/Fenix) for Android, and you could try using one of those instead, or rolling back to the final Fennec APK (I’ve been continuing to use Fennec as my primary browser while using the beta and nightly release channels for test use because Fenix is still far, far too bugged in ways that make my daily use cases non-fucntional e.g. tab unloading from memory behavior stopping background media streams after five minutes and reloading pages almost every time a tab or the entire app is put in the background, wiping page scroll/playback position and any entered text and thus making browsing and media streaming all but impossible). - JohannVII

А вот ответ на вопрос «почему в Firefox for Android больше нету about:config» - Линк

По поводу about:config в Firefox for Android уже говорили в 2020 году:

You can access about:config in the Nightly build of Firefox for Android, available from the Google Play store. - Paul

In response to about:config not working in Android Firefox, I have been using the nightly version but just recently a behaviour has changed. - Ben Vost

Вот ещё сообщение 2021 года:

The Android version of Firefox Daylight (Fenix) blocks about:config on the stable/release version of the app. I’m not entirely clear on the design details behind this (partly becaseu the dev response actually makes no sense), but the claimed reason is that a lot of the elements exposed by about:config no longer function with the new engine, or function differently than they do on the desktop version or did in Fennec (why about:config would even expose or contain unusable toggles in the first place in an application that has supposedly been entirely rebuilt is an open question), so it is disabled entirely in order to prevent normie users from completely borking their installations (again, that doesn’t really make sense, since a reset to default should fix any issues caused by editing config options, and the about:config page tracks and displays which options have been changed from defaults, so that reason doesn’t match reality).

The about:config page is functional in the beta and nightly releases of Firefox (Daylight/Fenix) for Android, and you could try using one of those instead, or rolling back to the final Fennec APK (I’ve been continuing to use Fennec as my primary browser while using the beta and nightly release channels for test use because Fenix is still far, far too bugged in ways that make my daily use cases non-fucntional e.g. tab unloading from memory behavior stopping background media streams after five minutes and reloading pages almost every time a tab or the entire app is put in the background, wiping page scroll/playback position and any entered text and thus making browsing and media streaming all but impossible). - JohannVII

А вот ответ на вопрос «почему в Firefox for Android больше нету about:config» - Линк