Do you prefer the old style of "..." or just want the text to cut off without fading?

Either way, I think someone will need to write a custom style rule (for userChrome.css or the Stylish extension).

As a regular release user, I probably can't help with that myself.

See line 54:

• chrome://browser/content/tabbrowser.css

.tab-label-container[textoverflow]:not([pinned]) {
  mask-image: linear-gradient(to left, transparent, black 1em);
}

.tab-label-container[textoverflow]:not([pinned]):-moz-locale-dir(rtl) {
  mask-image: linear-gradient(to right, transparent, black 1em);
}

It seems you would be able to override those rules using either

mask-image: unset !important;
mask-image: none !important;

for each of the selectors.

https://developer.mozilla.org/docs/Web/CSS/mask-image

Setting mask-image: none will remove the fading. I can't make text-overflow: ellipsis work for the tab text.

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

.tab-label-container[textoverflow]:not([pinned]){
  mask-image: none !important;
}

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration