It work mostly well, but it will have issues reconnecting if the computer goes to sleep or if Firefox serivce get interutped in some way. The extentions does not seem to be listed on Proton support pages, so not sure either where to report these issues or find forums to discuss issues.

When it works it's relatively good, though lacking in features compared to the full-fledged mobile or desktop apps (eg. no ability to configure profiles such as "stealth"). Unfortunately it very frequently fails to connect, or will say it's connected but you can't actually load any web pages. Sometimes the extension itself will even say that it is having trouble connecting and needs to be reinstalled which does work...until the same problem happens all over again. If it weren't for the reliability issues it would be an easy 4-star extension, with the 5th star being reserved for feature-parity with the mobile and desktop apps.

Not available for Firefox on Android, so I have no choice other than to skip it.

The extension is not working at all at the time I am writing a review so I am forced to use the desktop app again which cuts my internet speed to 1/4th. Review will change if this is fixed. Edit:they fixed it

Extension has been extremely reliable for me over the years but over the past couple weeks the experience has really gone downhill, the VPN doesn't auto-connect on browser start, it loses connection very often and does not connect to a new server, and it often fails, outright saying I need to re-install the extension, which I have done a couple times now, to no avail. Not sure what they are cooking but I can't say I'd recommend the extension right, I will be sure to update my rating should this be a temporary spell.

If isp blocking proton site there is no way to use this extension because authorization is going before establish vpn connection.

Для начала работы VPN надо войти в аккаунт, а при попытке войти в аккаунт выдаёт ошибку: "При соединении с account.proton.me произошла ошибка. PR_END_OF_FILE_ERROR". Не рабочее дополнение

Firefox extension has no support for mobile.

This is a major issue for anyone looking to maintain their privacy on-the-go.

If I were to use a VPN app that connects me to my home server, I wouldn't have the option of using the native Proton VPN client as well.

The lack of mobile support for this extension is a serious issue.

It works, but it's completely useless. For whatever reason it always selects the Netherlands, which is extremely slow here.
Being able to pre-select the country is a must, I think.