I used Firefox until they lost site of their core target, and killed their mobile app's quality and extensions.
It doesn't on my phone.
Last I checked, Mozilla did everything in their power to prevent you from running desktop extensions.
If I may ask:
1. Which version are you using?
2. Where did you get that version?
3. What settings/process did you change/follow to make firefox accept all desktop extensions?
Please be as specific as you can. A link to a Mozilla resources. if you can provide it, would be even better.
Link: https://pine64.org/pinephone. See also: https://puri.sm/products/librem-5.