I'm pretty frequently faced with a situation where I have 20 or more tabs open, and I wish to close all but 3 or 4 of them.It's a pain closing them all one by one, while being careful about not closing the ones I need. So, I built an Add-on to solve this problem.
This add-on allows you to close many tabs with a single click. Select File->Close Multiple Tabs, or just press Ctrl+Q to open a dialog showing the list of tabs that are currently open. You can select the tabs you wish to close from the list, and click OK (or just hit Enter!) to close them. If all the tabs are selected, then you have the option of choosing between a blank page and the browser's homepage.This way you can get rid of all the tabs and start a new session, without having to, perhaps, restart the browser.
Tip: Use Select All and deselect the tabs you need when you wish to get rid of all but a few tabs (like 3 or 4 out of 20), and use Select None and select the tabs you wish to close if they are few in number. This way, you'll find this add-on really handy.
PS: This add-on is also featured on:
Someone was nice enough to tell me that there's an add-on called Key-Config, using which you can change the keyboard shortcut for this add-on, if that's a problem (it is if you are a Linux user, or have the FoxTab add-on installed).