What?

So... Chromium based browsers do this thing where with every new tab, each other tab gets smaller and smaller, till you open enough tabs that newer ones stop displaying after a certain number of tabs. Firefox said "nah", and just decided to add a scroll wheel onto the tab bar.

This turns off that scroll bar and makes the tabs get smaller and smaller, but doesn't hide new tabs from showing after a certain amount. So what ends up happening is tabs keep getting smaller and smaller till you cant even see what they contain, including the close tab button. So I advise to not open more than 30/40 tabs at one time or just go use the Overflow version. Its much better.

Turns Tab Overflow Off

Yea...

/* Tab Overflow          */
/* On By Default         */
/* Thanks u/-SZCMAWO     */
.tabbrowser-tab {
    min-width: 0px !important; 
}
.tab-content { 
    overflow: hidden !important;
}

Make Selected Tabs Have A Minimum Size

Haha! Another broken section thanks to turning tab overflow off! If you open a seriously large amount of tabs and have one selected without a minimum width, you wont even be able to see the tab's favicon and close icon. This makes it so no matter how many tabs you have open, your selected tabs will still be closable.

/* Make Selected Tabs Have A Minimum Size */
.tabbrowser-tab[selected][fadein]:not([pinned]) {  
    min-width: 50px !important;
}

Set Minimum Size For Hovered Non-Selected Tabs

Again, thanks to turning off tab overflow, if you have a stupid amount of tabs open, they become so small that the close button isn't visible to click anymore. This sets a minimum size for when you hover over a tab, so that you are able to close them without opening them.

/* Mimimum Size For Hovered Non-Selected Tabs */
.tabbrowser-tab:not([pinned]):not([selected]):hover[fadein]{ 
    max-width: 245px !important;
}