Recent Firefox versions show a fancy “top sites” overview page every time the user opens a new (empty) tab. This is supposed to allow quick navigation to your most often visited sites.
If you (like me) don’t like the lag (and related security issues) that this adds to opening new tabs then set the new tab behavior back to showing a blank page:
- Open “about:config” in the address bar
- Acknowledge the warning to be careful
- Search for “browser.newtab.url”
- Right-click, Modify
- Change the value from “about:newtab” to “about:blank”
Only after writing this blog post, I noticed that there is a Firefox help page that explains the same procedure.
Oleander Donovan's IT blog 
Thanks Oliver. I did find that annoying, good tip!