Add Support for Installing .xpi Add-ons on Waterfox Android

[RaspberryKitty1]

Is your feature request related to a problem? Please describe.
I'm frustrated that Waterfox Android does not currently support installing .xpi add-on files directly, even though this feature is available in Firefox for Android. This limitation prevents users from manually installing custom add-ons or add-ons not listed in the Add-ons Manager.

Describe the solution you'd like
I would like Waterfox Android to include the "Install add-on from file" option in its settings, similar to Firefox Android. Since Waterfox is based on Firefox, implementing this feature should not be overly complex, as the functionality already exists in the base code of Firefox for Android.

Describe alternatives you've considered
The alternative is using Firefox Android's Developer Mode to manually install .xpi files, but I prefer Waterfox for its unique features and philosophy. Having this functionality directly in Waterfox Android would streamline the process for advanced users and developers.

Additional context
In Firefox for Android, the "Install add-on from file" feature is accessible via Developer Mode. Here’s how it works:

1. Tap the Menu button and go to Settings.
2. Scroll down and select About Firefox.
3. Tap the Firefox logo around 5 times to enable Developer Mode.
4. Once Developer Mode is enabled, the "Install add-on from file" option appears in the settings.

Given that Waterfox shares a Firefox base, it seems feasible to include this feature with relatively minor adjustments. It would greatly enhance Waterfox Android's usability for power users.