Depending on your settings (Firefox Menu > Options > Options > Advanced > Update Tab) you will be promoted within the next the 24-48 hours to update or can manually check via Firefox Menu > Help > About Firefox. You can also download via getfirefox.com and install Firefox 18 this way.
The next scheduled release of Firefox will be with Firefox 19 on Tuesday, February 19, 2013.
The problem with notifying when closing multiple tabs still hasn’t been fixed.
This is when right-clicking and selecting ‘Close Other Tabs’? I haven’t seen anything on that in a while, I do some digging around later and see what I can find.