Lol. MacOS is based on BSD and Mach which are open source, but MacOS is still proprietary. Any normal modern browser syncs anything between devices and has a builtin password manager (which supports sync as well). Even stock Firefox is customizable with countless themes and vertical tabs and has a decent adblocker. No idea why you need calendar in a browser though.

And, finally, a quote from Vivaldi's EULA, showing its high attitude to being FOSS:

7\. Without limiting the foregoing, you are neither allowed to (a) adapt, alter, translate, embed into any other product or otherwise create derivative works of, or otherwise modify the Software ; (b) separate the component programs of the Software for use on different computers; (c) reverse engineer, decompile, disassemble or otherwise attempt to derive the source code for the Software, except as permitted by applicable law; or (d) remove, alter or obscure any proprietary notices on the Software or the applicable documentation therein.