I use Firefox for Linux Mint mint-001 - 1.0 130.0.1 (64-bit) on Linux Mint 21.1 and have recently installed the latest version of the MainWP browser extension.
At the same time, I also installed it in Google Chrome on the same system.
It works well in Google Chrome (I expected more from it) but since installing it, websites have not been displayed correctly in Firefox and the MainWP browser extension has not finished synchronizing for hours.
We are aware of the layout breaking of some websites in Firefox caused by the MainWP browser extension.
This will be fixed in the next release of the extension.
For Chrome - if your synchronization is not working, please try creating a new REST API key, and in the Legacy API Credentials section, make sure to enable Enable MainWP REST API v1 Compatibility.
Should the issue persist, please open a private Help Desk ticket so we can collect some additional information & investigate further. https://mainwp.com/my-account/get-support/