You may have accidentally zoomed webpage(s). Reset the page zoom on pages that cause problems.

• View -> Zoom -> Reset (Ctrl+0/Command+0 (zero))

• https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages

It's not the font size on the reading pages, it's the toolbar and tabs

I will sure appreciate help

You can look at this thread:

• /questions/1384204 Since the last update, the Firefox screen does not fit my system setting(125%)

If setting browser.display.os-zoom-behavior = 0 doesn't work for you then you can create a new Number pref on the about:config page and set its value to 100. Paste ui.textScaleFactor in the search bar and select the Number radio button and click the '+' button to add the new Number pref, enter the value (100) and click the blue OK button to confirm.

• about:config => ui.textScaleFactor = 100

You can open the about:config page via the location/address bar. On the warning page, you can click "Accept the Risk and Continue" to open about:config.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox