I have used Protonmail merrily for many years, and trust it quite a lot, so when this service popped up I was very promptly in.
The very second website I used after installation was such a spectacular disaster that I had to remove the add-on entirely.
I will TRY to explain (plus superficially mention a couple other things I found lacking).

1. I don't love the large pop-up window asking for something on the side. I IMAGINE it asked if I wanted to save my password for this site, or maybe if I wanted to implement the password it had already stored from the CVS import I had performed minutes earlier. There was no way to know because most of the right-hand side text was clipped, and it would not fill the space even if I reduced the zoom level of the page.

2. Installation process... not great. Too unclear on the results. In my case I could never plainly see if the add-on was already working unless I checked via the Firefox manager.

3. The really egregious problem took place as I was trying to log-in into my bank's website.
My bank has a system that includes 3 pages for that purpose.
1st page you enter your card. 2nd page your password. 3rd page you scan a QR code that a physical token reads and gives you a numerical transitory PIN to be entered in some fields.
It was on this third page that the catastrophic behaviour of the add-on manifested.
The PIN is 8 digits long. Each digit is entered in its own separate space (there are 8 different tiny areas to enter each number).
The 1st and 8th areas showed automatically the icon of ProtonPass. And nothing else could be input into those spaces.
So from 8 digits, I could only enter 6.
Since those numbers are time sensitive, the only way to solve the situation was to deactivate the add-on.

You can imagine how useless this nice idea quickly turned for me.

I hope I was able to explain the situation because my intention is not to simply to berate this tiny piece of software, but to actually help make it better.

Best wishes with the development, very likely I'll come back some months later to check it again.

I hope the Proton team is putting a lot of effort into this extension because currently, it's not working well at all. I'd say that it only offers to autofill my info on about 50% of the sites I've tried to use it on. When I enter new login info for a website, it pretty much NEVER offers to remember it. I would submit bug reports, but I don't want to take the time because it doesn't work more often than it does. I really wish there was a standalone app for Windows instead of only the browser extension because I'd like to be able to use it for passwords that are outside of the browser as well. I also worry that there could be security vulnerabilities that are due to the browser and extension ecosystem. I'm a paid subscriber and rather disappointed, at the moment.

A bit buggy, but I have hopes for the future of this extension.

In it's current state, passwords fail to autosave, autofill is shaky and it is probably a good idea to use in conjunction with something like Bitwarden. Hopefully they will resolve this swiftly.

So far, it doesn't work on most websites. I mean, it doesn't show the Proton Pass icon and don't autofill.