The 1P button did not appear in Firefox, despite repeatedly asking 1Password to do it, and receiving no notice of failure.
After investigation, I have figured out it was because Firefox was not directly in the Applications folder, but in a subfolder instead. When installing extensions, 1Password just remains totally silent as to which browsers are actually found, this is not very user-friendly!
Under Mac OS X, it is not a requirement for an application to be directly in the Applications folder, therefore this qualifies as a bug.
It is my own practice to put all "extra" applications in subfolders of the Applications folder, just like the "Utilities" folder.
This way I can separate the applications that I have bought, from the Freeware, and from the default applications that come from Mac OS installation.
Additionally, if an application come with a PDF manual, or with some "Extra stuff", I group all this in yet another sub-folder to keep it all organized. It's all natural to do it like that.
Even 1Password itself is located in a subfolder on my Mac. It works fine with no complaint, except that it cannot find Firefox!
So… I suggest adding a manual "locate" function:
The Advanced box could list the browsers ACTUALLY found instead of showing all compatible browsers in existence (user may be confused with those unknown browsers, wondering whether they exist on his/her mac).
Then add a button to let the user find more browsers manually if necessary.
Finally, a user feedback to signal success after installation on each browser would be nice too!
Incidentally, I think that 1Password could recursively extend the search to sub-folders of Applications, that seems so natural… In my mind, a file I place in a sub-folder of the Applications folder is technically in the Applications folder as well, just like a book placed in a drawer in my house is indeed in my house. That's the way the folder metaphor works...