Could you clarify whether this problem is on Firefox for your Mac or another program/system, such as Thunderbird for your Mac or a FirefoxOS phone? Want to make sure your question ends up in the right queue.

If the problem is in Firefox, you can quickly reset the zoom level for the site displayed in the current tab to its default by either:

• "3-bar" menu button > click the percentage between the - and + buttons
• on the keyboard, Command+0 (that's a zero)

More on using zoom in Firefox: Font size and zoom - increase the size of web pages

My apologies, I got to support through the Thunderbird portal. This problem is in Thunderbird for Macbook Pro, OS x. Type size on screens for email folders, ie, inbox, drafts, or others created folders is very small. When looking at lists of emails subject lines in folders, very difficult to read and "Zoom" feature is disabled. Only when email is opened, "Zoom" is enabled and type size can be adjusted. Sometimes I make mistakes and open wrong emails that have similar names. Have to get magnifier to read the lists easily. Thanks for any help. C Sanders

I thin you will find it is this add-on your looking for https://addons.mozilla.org/en-us/thunderbird/addon/theme-font-size-changer/

Install instructions http://chrisramsden.vfast.co.uk/3_How_to_install_Add-ons_in_Thunderbird.html

Worked perfectly!!!!