This is a little difficult to answer because the starting point in Firefox 22 and later is the zoom level set in Windows ("Text Size" setting, often 125% on newer systems). So if you reset the zoom level on a page to default (Ctrl+0) and open the following test page, the zoom level (window.devicePixelRatio) may be higher than 1.0:

http://dev.jeffersonscher.com/resolution.html

The following should be understood as an adjustment applied to that base level.

By default, the enlargement options are:

1.1 => 110%
1.2 => 120%
1.33 => 133%
1.5 => 150%
1.7 => 170%
2 => 200%
2.4 => 240%
3 => 300%

By default, the reduction steps are:

.9 => 90%
.8 => 80%
.67 => 67%
.5 => 50%
.3 => 30%

To add or modify these steps, you can edit this list in Firefox's about:config preferences editor. Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box that appears above the list, type or paste zoom and pause while the list is filtered

(3) Double-click the toolkit.zoomManager.zoomValues preference and edit as desired. Since it's a little hard to read, you might want to copy it to a word processor or other program where you can zoom the text, edit it there, then copy back the revised version.

Note #1: After OK'ing this change, it will take effect in any new windows you open. Existing windows/tabs appear to remember the setting in effect at the time you opened them. (To move a tab to a new window, right-click the tab on the tab bar > Move to New Window.)

Note #2: if the preference becomes a mess, you can right-click > Reset it to restore the default.

Oh, I should mention that you can relocate the zoom controls from the menu panel to the toolbar for easier at-a-glance reading.

Near the top of the "3-bar" menu panel you will see something like:

- | 100% | +

To move that to the end of the toolbar, right-click it and choose Move to Toolbar.

To move it to a different location on the toolbar, use the Customize feature. See: Customize Firefox controls, buttons and toolbars.

However, when reading the percentage on that zoom level reset button in the center, bear in mind that it is relative to your base zoom level, and not the whole story in itself.