For a long time, my password practices were atrocious; I reused the same password just about everywhere and didn't change them for years at a time. I knew that this was bad, but there was no good solution for firefox on linux. KeepassX had limited support, but it was rather disgusting both in the semantics of an overly complicated database and the dependency on Mono.
That changed with zx2c4 pass, the first password manager that is simple, secure (built on gpg) and built for unix systems. It doesn't require a big, ugly gui somewhere, access control is automatically delegated to the system gpg agent, which uses pinentry.
Of course, the usability of this is greatly reduced if you have to copy your passwords from a terminal, which is why this addon is so important. I imagine that most linux powerusers like myself use firefox, and finally we have a password manager integrated with firefox that doesn't completely suck.
I've been using this addon for about 6 months, and have migrated all my credentials to its store. I've watched it go through a number of upgrades; at first it was barely usable and often broke entirely to the point where I would give up and just paste from a terminal, but since the most recent upgrades, it has greatly improved in interface and stability to the point where it is a joy to use.
It's still not quite perfect: some websites call their their input elements by strange ids which breaks the autofill capability. You can add element names in the addon options, but those are global across all credentials; it would be nice to see support for per-credential element names. Also, matching on domain and url is still sometimes a bit wonky; ironically, trying to log into addons.mozilla.org grabs my firefox login credentials by default, I have to manually select the correct set.
In the scheme of things, the problems are minor for the great increase in usability this addon provides, which is why I rated this five stars. Also, based on the pace of development so far, I fully expect the user experience to improve in coming releases.
I don't expect this to have mainstream appeal, but if you use linux and aren't afraid of the terminal, this is the password manager for you. I believe this should also work on OS X and might even work on Windows with Cgwin, though I haven't tested either. Really, the limiting factor is the availability of the underlying program pass, which depends on the availability of bash, gpg and the usual system utilities.
Finally, I would like to add that it would be very nice to have an android version of this addon, though I am unfamiliar with android development and unsure of how feasible this is.