support setting default ua for mobile firefox

[zdking]

I want to make default ua can be set instead of just hard-code to mobile.

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When I access some websites using mobile UA, they recogized this and just render a tidy webpage to force user downloading their apps. But I don't want to do this. After I set my ua to PC by 'Desktop Site' option, they render the page properly.

Currently, I can set this ua to PC manually for each webpage I visit. But setting 'Desktop Site' always fails when app killed and start again, and the ua cannot be inherited when I start another webpage for a PC-ua one.

What I want is to allow default ua setting and also keep the ua's value on the conditions above.

[zdking]

Here's the information displayed on 'About Firefox'

105.2.0 (Build #2015907747), 01fbfd637+

AC: 105.0.8, 658c2d239f

GV: 105.0.2-20221003135607

AS: 94.1.0

And my device info:

Android 12,  kernel version: 4.19.157-perf-ga2f6140ae6bd, Qualcomm SnapDragon 865 (8 cores)