It could happen to you that you accidently closed a tab in firefox which had some web page that you want to read again, for this firefox has embedded special feature in firefox called recently closed tabs. You can re-open any of the recently closed tabs in firefox if you go to File Menu >> History and click recently closed tab link to re-open it again (as shown in the image below)
Still if you want to do something permanent about this problem then, you can use the firefox addon called PermaTabs Mod ( via Ghacks.net on Make Tabs Permanent In Firefox ) this firefox addon lets you make tabs permanent in firefox which cannot be closed accidently no matter what you click, you cannot even close these tabs by actions like Close Other Tabs. These tabs which you specify as permanent will open in every session of firefox you start, you can change the colour of these tabs so that a new user gets to know that these tabs are special.
Another way to make tabs such that users can’t close a tab simply by mouse click is by hiding the cross sign ( via HowToGeek.com on Quick Tip: Remove Close Button from Firefox Tabs ) in this way you hide the close cross on firefox tabs by simple about:config firefox hack. Here is how you can do it.
1. Open Firefox
2. Type about:config in the address bar and press enter key on the keyboard
3. Click the button I’ll be careful, I promise!
4. now type browser.tabs.closeButtons and double click the value of the key below to 0 the default value is 1 – after setting the value to 0 you wont see any close buttons on the tabs but you can still close them using mouse middle click and pressing ctrl+w
Tip: Another way to hide the close button is with Tab Mix Plus Free Firefox addon.