The solution I would recommend is the Tab Mix Plus add-on
https://addons.mozilla.org/en-US/firefox/addon/tab-mix-plus/ which allows you total control over tabs and so has far more settings than just what you want.
I've been using it for far too long to be able to remember what changes you might notice just because of its default settings when installed but hopefully it won't impose any changes on you initially.
If you do decide to use it then go to its setting, select Display, then Tab and in the bottom are of those settings you can set minimum/maximum widths (e.g. 100 / 1000) and a flag so that the tab width fits the tab title.