Say I follow this process and build my own copy of FF the binaries produced at the end won't be signed by Mozilla and will fail your code signing check. If I'm concerned enough about security that this might be a problem, as with Waterfox I can build my own copy of FF from the public source tree - Mozilla even provide a handy-dandy compilation guide.

What makes Mozilla trustworthy? Firefox is fully open-source, but we can't be certain that the public binaries Mozilla provide are unmodified from the public code. My personal browser preference aside, the inability to disable this check doesn't seem to result in a real improvement in security. Either option defeats the entire purpose of using 1Password or its' competitors if the pure manual copy-and-paste option was practical, the browser extension wouldn't need to exist. With the inability to disable the signing check, users of non-mainline browsers are effectively forced to either return to using several complex but not unique passwords, or store copies of their 1Password passwords in Waterfox's password vault. There's a small risk that the developer could choose to intercept my credentials, but the code is open-source - I suspect any such feature would be both hard to find & easily identified - and I can build it myself from the public source tree, ensuring I know exactly what I'm running. Until this change was implemented, I could flip a single setting to make 1Password work with Waterfox. The FF55.0 changes forced a profile fork, so switching back to vanilla Firefox at this stage is a tedious & painful process leaving me with a browser that uses more resources, crashes more often, and removes features I value. Prior to Mozilla's changes in 55.0, FF and WF shared profile information & could be swapped between at will.

'Sticking to Firefox for now' isn't an option. It uses less memory, it crashes less often, it runs faster, and it's just generally better. Waterfox is (in my experience and opinion) a massively superior browser to Firefox, especially now as Firefox is dropping support for a number of older incredibly useful extensions (which, in some cases, are simply no longer being developed and thus will never be replaced).

I'd like to add something to this if I may (and add my vote for Waterfox support!)

Mozilla waterfox