You also have a list of all open tabs available from the dropdown arrow at the far right, but the tab width can be changed in about:config with browser.tabs.tabMinWidth.

If you're looking for information on this preference, it went away between FF4 and FF57(?) but should be back and working now so you'll see a lot of old information about it being removed.

Edit: FF58, https://bugzilla.mozilla.org/show_bug.cgi?id=1404465