Hi,
Please try pressing '''Ctrl''' + '''0''' (zero). This resets the zoom. Alternatively View ('''Alt''' + '''V''') > '''Zoom''' > '''Reset'''.
[https://support.mozilla.com/en-US/kb/Keyboard%20shortcuts Keyboard Shortcuts]
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Useful links:
[https://support.mozilla.com/en-US/kb/Options%20window All about Tools > Options]
[http://kb.mozillazine.org/About:config Going beyond Tools > Options - about:config]
[http://kb.mozillazine.org/About:config_entries about:config Entries]
[https://support.mozilla.com/en-US/kb/Page%20Info%20window Page Info] Tools (Alt + T) > Page Info, Right-click > View Page Info
[https://support.mozilla.com/en-US/kb/Keyboard%20shortcuts Keyboard Shortcuts]
[https://support.mozilla.com/en-US/kb/Viewing%20video%20in%20Firefox%20without%20a%20plugin Viewing Video without Plugins]
[http://kb.mozillazine.org/Profile_folder_-_Firefox Firefox Profile Folder & Files]
[https://developer.mozilla.org/en/Command_Line_Options#Browser Firefox Commands]
[https://support.mozilla.com/en-US/kb/Basic%20Troubleshooting Basic Troubleshooting]
[https://support.mozilla.com/en-US/kb/common-questions-after-upgrading-firefox-36 After Upgrading]
[https://support.mozilla.com/en-US/kb/Safe%20Mode Safe Mode]
[http://kb.mozillazine.org/Problematic_extensions Problematic Extensions]
[https://support.mozilla.com/en-US/kb/Troubleshooting%20extensions%20and%20themes Troubleshooting Extensions and Themes]
[https://support.mozilla.com/en-US/kb/Troubleshooting%20plugins Troubleshooting Plugins]
[http://kb.mozillazine.org/Testing_plugins Testing Plugins]
