Features workspaces for Firefox similar to Opera's, accessed via keyboard shortcuts, persistent across restarts.
Operation
alt + 1-9 - change current workspace
alt + g - merge and reset all workspaces
Recommendation
Change that closing the last tab automatically closes the window - in the search bar, enter about:config - there, search for browser.tabs.closeWindowWithLastTab - set it to false, by double clicking true
Other Notes
Absolutely no data is collected, hence neither shared nor sold.
