Sometimes it is useful to restart Firefox but keep all the previously-open tabs and windows.
Unfortunately, simply closing the browser will not do as the next time it is started, it will simply show your default start settings. This is blank page in my case.
To assist with this, there was a couple of extensions, like Restartless Restart or the Restart Button, but they all stop working after some time due to Firefox updates.
The solution is available in Firefox by opening the Developer Toolbar (link) using Shift+F2 and typing "restart" as the command.