How to change TAB font size in ANY BROWSER

Solution 1:

You can edit your Firefox profile's userChrome.css or use the Stylish add-on to define the following CSS rule:

#tabbrowser-tabs
{
    font-size: ... !important;
}

Where ... is replaced with a valid font-size value.

Example:

#tabbrowser-tabs
{
    font-size: 20px !important;
}

Video Demo: http://www.youtube.com/watch?v=BAhzZuhN61E

PS: If you want to also edit the font size of the List all tabs popup menu, the CSS selector is:

#alltabs-popup menuitem

PPS: Once you've increased the size of your tab's text, you can also increase the width of each tab to accommodate:

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

Where ... is replaced with a valid min-width or max-width value. Play around with these values to get different effects! Note: Pinned tabs will remain the same size.

Example:

.tabbrowser-tab:not([pinned]) 
{
    min-width: 400px !important;
}