I would start by resetting the zoom level. For that you can use any of these:

• Ctrl+0 (that's a zero)
• "3-bar" menu button, the click the % number between the - and + buttons
• (menu bar) View > Zoom > Reset

From that point, you can adjust further as needed.

More information: Font size and zoom - increase the size of web pages

None of these options either are unavailable they Don't work. The problem is with one web page only.

Type about:preferences#content<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

You can check for corrupted and duplicate fonts and other font issues: http://www.thexlab.com/faqs/multipleappsquit.html - Font Book 2.0 Help: Checking for damaged fonts http://www.creativetechs.com/iq/garbled_fonts_troubleshooting_guide.html

Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies
  Warning ! ! This will log you out of sites you're logged in to.

Type about:preferences<Enter> in the address bar.

• Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.
• Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.

If there is still a problem, Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Then restart.

You can't zoom the size of text on any website? Or that one site is presenting content in a format that is unzoomable? Do you want to mention what site it is?

The web site / page is Experience Project. I was reading a post on it when the page suddenly shrunk to the point that the text was so small It was unreadable.

the

Please provide a public link.

One possible reason for a sudden zoom level change while reading is if you are scrolling the page with the mouse wheel but you happen the press the Ctrl key while doing that. To reverse the change, hold down the Ctrl key and move the wheel in the opposite direction.

If an input field or text area pr plugin has focus then the keyboard won't work.

Reset the page zoom on pages that cause problems.

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

You can tap the Alt key or press F10 to show the Menu Bar

• http://kb.mozillazine.org/Zoom_text_of_web_pages