Because unlike other browsers Firefox will actually search existing open tabs and present those as possible results (and open the tab if you choose it).
I have hundreds of tabs open at a time. Instead of searching for something, then going to the Google page, clicking and waiting for it to load, in Firefox I search for what I want in the bar, it presents the tab as a result and opens it instantaneously.
In addition with the vertical tab bar extensions I can see a list of about 40-50 tabs open at a time, using the additional horizontal space monitors provide that web pages don't use to keep an easily visible list of tabs.
I have hundreds of tabs open at a time. Instead of searching for something, then going to the Google page, clicking and waiting for it to load, in Firefox I search for what I want in the bar, it presents the tab as a result and opens it instantaneously.
In addition with the vertical tab bar extensions I can see a list of about 40-50 tabs open at a time, using the additional horizontal space monitors provide that web pages don't use to keep an easily visible list of tabs.