From MozillaZine Knowledge Base
To fit more tabs on the tab strip, Firefox shrinks each tabâ€™s width. This preference determines the narrowest a tab can become before the tab strip becomes scrollable to handle the overflow.
Possible values and their effects
Any positive integer value of pixels. Browser tabs may shrink to this value, but may not become any narrower. Default value is 100. Note that due to the design of the tabbar, the minimum width for a tab is 38px, so using a value less than 38 will not have any effect.
Changes made to the value using about:config only take effect on windows opened after the change.
First checked in
Has an effect in
Not present after