"Begin disabling installed extensions still using Manifest V2 in Chrome stable"

Chrome to Firefox is a relatively easy switch, especially for those that don’t depend on Google sync. The main sources of friction for me were the lack of a good profile switching UI (solved with a browser extension that mimics the Chrome menu), and weird security requirements for homemade extensions (IIRC if you want to have the extension persist after restarting Firefox, you need to sign the extension, which is a pain)

For users switching from Arc, there is no good alternative, but Firefox with Sidebery and custom CSS comes close.

I don't know if this is what you meant, but as an alternative to profile switching, there are Multi Account Containers [1]. It allows assigning a container to each tab, and the containers are isolated from each other. If you have an MS or Google account for both work and personal, you can open them at the same time in different tabs.

[1] https://addons.mozilla.org/en-US/firefox/addon/multi-account...