You can reset Zoom - CTRL+0 (CTRL+zero) - see: https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages
Zoom can be found by pressing the ALT key and then form the temporarily displayed Menu Bar (top left) choose View >Zoom.

You can use this add-on; it will display zoom %-age in the Addon Bar:

• Default FullZoom Level - https://addons.mozilla.org/en-US/firefox/addon/default-fullzoom-level/
  • click Addon Bar DFZL and choose zoom level
  • point to Addon Bar DFZL and use scroll wheel up/down to zoom/un-zoom
  • right-click Addon Bar DFZL to change from text-only to full-page zoom
• Info about Addon Bar - https://support.mozilla.org/en-US/kb/add-on-bar-quick-access-to-add-ons

I do not want to font-size-and-zoom-increase-size-of-web-pages. I want to know how to determine if the browser resolution is at 100%

Using CTRL+0 (CTRL+zero) resets the font size and/or graphics to 100%. Firefox does not have a default display of the percentage. The add-on suggested will display that font (text) percentage ("T100%") or full page percentage, including text and graphics, ("F100%") in the Addon Bar.

Zooming is about changing the size of the font (text zoom) and/or text and graphics (full page zoom) on a displayed web page. Web page design may over ride your default settings for the typeface/font used for that page. From the first paragraph of the article above:

"Zoom is a feature which allows you to increase or decrease...the size of all text displayed in a web page (Text Zoom)..."

Zooming text is increasing the default font size by a percentage for readability.

The font size used to display web pages is determined by your settings or by the web page designer in Options > Content > Fonts and Colors section and the "Advanced" button in that section. I would suggest you not make changes there as web pages may not display properly after your changes. For best display of a web page, always allow the web page to choose the font and typeface by leaving checked "Allow pages to choose their own fonts, instead of my selections above" in the "Advanced" section. Even with web page determined font/typeface, you can increase the size of the text and/or full page with zoom and reset using CTRL+0.

• See - http://support.mozilla.org/en-US/kb/settings-fonts-languages-pop-ups-javascript

Thank you much. Appreciate it! Cynthia