I installed Vivaldi, and moved shortly back to Firefox. The two things that did it:
1. I accidentally saved the wrong password to a site. When I went to fix it, it said I needed to login to my Google account to change my stored password.
Wait, wat?! You are sending my passwords to Google, unencrypted, without telling me?!
Thai is not acceptable, aside from - What else are you sending?
1b. I see that it saves non password fields without my asking me to save them, and sending that to Google as well. Hmmmmm.
2. It has twice crashed and lost all my open tabs (yes, I have a tendency to keep many open tabs).
Also, for all I know, they are selling my bank account login. Research into the history of Vivaldi (semi fork of Opera) and its dev team left me unsatisfied who they are and that I can trust them.
It probably sends less data to Google than Chrome (which gets every single page, its contents, how long it was open, etc. but is hardly a contender if you want a browser and not just spyware.
Vivaldi doesn't save passwords using Google. Did you perhaps click on a help link that sent you to a Google support page for Chrome?
I see that if you click the password icon the address bar and click the "Manage passwords" button it opens the default password settings page inherited from Chromium (vivaldi://settings/passwords) which includes a link to a support page for Google Chrome (https://support.google.com/chrome/?p=settings_password), but not all the information on that page is applicable to Vivaldi. In particular, Vivaldi doesn't use Google but rather its own account system for browser sync (which is optional, same as Firefox and Chrome).
That appears to be a bug, since that legacy Chromium password settings page isn't Vivaldi's normal password settings page (vivaldi://settings/privacy/). But it doesn't seem malicious.
Incidentally, the built-in password manager in Vivaldi (as well as in Chrome and other browsers based on Chromium) doesn't let you manually edit an existing password, whether or not you use an account to sync them. You can only update an entry by signing into a site with a new password and confirming the password change if the browser detects it, or deleting the old entry and saving a new one. A limitation compared to Firefox's password manager, though I do appreciate the native ability to generate random passwords in Chromium-based browsers. I hope Firefox and Chrome copy each other in those regards.
I haven't experienced any crashes with Vivaldi, though I don't use it as much as other browsers such as Firefox so perhaps I've just been luckier.
It was founded by Norwegian developers who left the original Opera (either due to switch from the old Opera browser to the new interface, or because the company was sold to Chinese investors). I do wish they were more open with the source code, but anyone who was comfortable with using the original Opera back in the day should be okay with Vivaldi. More so than the current Opera, I think, which I still see many people using due to brand recognition I assume.
1. I accidentally saved the wrong password to a site. When I went to fix it, it said I needed to login to my Google account to change my stored password.
Wait, wat?! You are sending my passwords to Google, unencrypted, without telling me?! Thai is not acceptable, aside from - What else are you sending?
1b. I see that it saves non password fields without my asking me to save them, and sending that to Google as well. Hmmmmm.
2. It has twice crashed and lost all my open tabs (yes, I have a tendency to keep many open tabs).
Also, for all I know, they are selling my bank account login. Research into the history of Vivaldi (semi fork of Opera) and its dev team left me unsatisfied who they are and that I can trust them.
It probably sends less data to Google than Chrome (which gets every single page, its contents, how long it was open, etc. but is hardly a contender if you want a browser and not just spyware.