You can reduce the minimum width of tabs, which "out of box" is 76 pixels in Firefox 57 and newer, and before that, was 100 pixels. For your version (Firefox 51? maybe 52 ESR?), you would need to use a custom style rule and apply that using either a userChrome.css file or the older version of Stylish (before version 3).

Thanks for your reply.

I thought there might be a configuration line I can edit at about:config.

Anyway, about stylish, is this what you mean? https://addons.mozilla.org/en-US/firefox/addon/stylish/versions/

How can I apply that new settings.

My Mozilla's version is 51.01.

Here's an example of a user style you can apply using Stylish (version 2.11 or earlier in Firefox 56 or earlier):

https://userstyles.org/styles/135164/firefox-minimal-browser-tab-width

A minimum width of 40 pixels might be too small for your taste; you can edit the style on the Add-ons page after installation.