It is great to have this ability again!!! Thank you!
However, there are some problems:
a. If you close a "locked" tab, and then undo it, the lock icon is not there anymore
b. Why doesn't the extension add menu options (tab menu, context menu, etc) to lock the current tab in addition to the toolbar button?
Also, this extension would be much more useful to me if it could "protect" tabs as well:
1. make the Close Tab button disappear
2. remove the Close (current) Tab option from the menus
3. display a dialog when attempting to close Firefox when it has a protected tab
4. ignore protected tabs when using other tab-closing functions (like close-other-tabs, close-tabs-to-right, close-tabs-to-left, etc)