May be fine if you don't log in but don't expect it to uphold the same privacy standards as an official app. I thoughtlessly used it, logged in and then realised I may have just handed my Proton credentials to an untrustworthy Developer - take note this is their only add on and their account was only made last year, so there is no way to do a background check. Another thing is they didn't try to be explicit about it not being an official app - just a few red flags to keep in mind.
Stay safe out there everyone and be wise.

This is NOT an official Proton or Lumo extension. The publisher states that in their description, but if someone does not take time to read all that, they would think from the title that it is official.