You can rename (or delete) the content-prefs.sqlite file in the Firefox profile folder to reset the page zoom for all websites.

Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the above file. Then rename or delete it. Restart Firefox.

Well, I did find the file and renamed it mud, as it didn't appear to want to let me delete it. Sadly, it did not stop the out of control zoom feature. I am getting genuinely frustrated with this.

Zoom can also be done using <Control> mouse wheel.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Could you describe the automatic zoom in more detail? Does it occur without you having the touch the mouse or keyboard, or at some other time?

Some pages have a "responsive" layout and will adapt to your screen width and resolution, but that' typically occurs just when you load the page or resize the window, and not at odd times.

For reference, the built-in ways to change the zoom level are:

• toolbar buttons ( - | 100% | + )
• keyboard shortcuts (Ctrl+-, Ctrl+0, Ctrl++)
• Ctrl + mousewheel

(Unless there's some I don't know about!)

I have the same problem and I have to keep using the - 100% + buttons over and over! I just want to be able to make things stay the same size, 100% is good.

https://support.mozilla.org/en-US/kb/changes-toolbars-and-window-sizes-are-not-saved You may have corrupt xulstore.json or Localstore.rdf file.

Close Firefox. Open your file browser to this hidden location:

• Mac: *~/Library/Application Support/Firefox/Profiles
• Linux: *~/.mozilla/firefox/Profiles
• Windows: %AppData%\Mozilla\Firefox\Profiles
• *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles
• Windows XP: C:\Documents and Settings\<user>\Application Data
  \Mozilla\Firefox\Profiles\

Search for the above files. Then rename or delete them. Restart Firefox.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

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

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

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

In addition to the above (a thing to try) :

Type in the address bar about:config (press Enter) (promise to be careful, if asked)

Type and look for the preference : zoom.maxPercent and set its value to 100

Type and look for the preference : zoom.minPercent and set its value to 100

Close and restart Firefox.