Hello, and welcome!
/usr/local/share/doc/pkg-readmes/firefox recommends not changing the files in /usr/local/, instead making local copies in /etc and editing them there. Otherwise, you will need to revise the files each time you update the package.
Quote:
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Files containing pledge promises and unveil paths and permissions for each type of process are located in /usr/local/lib/firefox/browser/defaults/preferences/. Each file can be overridden by copying it to /etc/firefox/ and modifying it.
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There are two recent forum threads that discuss this subject, which might help.
One.
Two.