Hi. Assuming you mean the width, enter about:config in the address bar and change the value in the following preference. browser.tabs.tabMinWidth The minimum you can set is 50.

https://support.mozilla.org/en-US/kb/about-config-editor-firefox

That would involve overriding this rule via code in userChrome.css. Note that you need to add the !important flag.

.tabbrowser-tab:not([pinned]) { max-width: 150px !important; }

See also:

• https://searchfox.org/mozilla-release/source/browser/base/content/browser.css#206

.tabbrowser-tab:not([pinned]) {
  flex: 100 100;
  max-width: 225px;
  min-width: var(--tab-min-width);
  transition: min-width 100ms ease-out,
              max-width 100ms ease-out;
}