Then please answer my question, Is it safe to use?
I don't know, do your own research about this.
I prefer using open source as much as possible, but outdated software with any license can also be problematic sometimes.
Crazy idea: would it be possible to encrypt notes (from the browser) and upload it to your server?
Or simple export/import/backup feature in local files.
Any time you uninstall extension or use different browser, you just import backup files and it's all done, similar like with bookmarks.
No need for cloud (someone else's computer).