Firefox extensions are installed as .xpi files, which can be opened with any archive manager. The BPIP source code is entirely open source and viewable within this archive.
BPIP website is clearly NOT the same thingas BPIP extension.
Even if you can download and edit .xpi files, there are different licenses with all rights reserved, so you can't edit them just like that.
I mean you can, but it's not ethically correct thing

btw please show me where you saw open source license on BPIP?
https://bpip.org/