How can I manage computers using my Firefox Sync account?
Firefox Sync lets you take all your Firefox data (such as your bookmarks, history, passwords and open tabs) with you wherever you go.
You just set up Firefox Sync on your main home computer and your Firefox data will be securely stored on Mozilla's servers and synchronized automatically from then on.
Then, when you set up Firefox Sync on your work computer (or even your phone running Firefox), your Firefox data will be automatically synchronized with your main home computer.
There is no removing computer from the account manager.
If you want to manage the computers that synchronize with your main computer just remove FireFox Sync from Firefox Addons
on those computers and they will not synchronize automatically anymore.
More information to Mozilla Firefox support Site: How to sync Firefox settings between computers
From this Mozilla Support thread :
Simply sync all your computers, then go into the Weave settings, and hit the button that says roughly: "reset all computers with this computer's data."
After that, the other computers will be removed from the tab list, and you'll be pretty much all set.