Getting zillions of tabs or new browser instances?
Killing them all by hand every now and then?
Small trick which tells Firefox to put all URL requests in the same tab, reusing one tab.
in the URL type: about:config
find the property browser.link.open_newwindow
change the value to 1
Follow up tip: Keep some Firefox fork/clone (or whatever) installation which you use just for development, and apply this setting to it. This way you don't mess up your Firefox for normal surfing. I keep swiftweasel in /opt/swiftweasel