Follow Mastodon User от Joshka

This extension adds a button to the toolbar that will popup a window to follow the Mastodon user on the current page. This is mostly to scratch an itch where some mastodon pages make you have to copy the username, then search for it in your own instance, then click follow there once the search loads.

To use a server other than mastodon.social, go to the settings page and enter the server address including https://, omitting any trailing slash.