Ever had more than one Firefox windows open, with tabs and ideas scattered all over the place? What a mess! The good news: it’s easy to fix. You’ll need the Tab Mix Plus extension.
Ctrl + Shift + M
will join all open windows into a single one. This behavior is configurable in Tools > Tab Mix Plus Options > Tab Merging.
Having the option to detach / reattach a single tab would also be useful, but I couldn’t find a quick way to do it, except manually copying the URL and pasting it into a new window.