Recently I decided to switch to Firefox and found a really annoying problem. The I-Beam appears next to non-editable texts if I click next to them. This breaks the Home, End and PgUp / PgDn keys.
How can I prevent this?
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This is called caret browsing. It is a Firefox feature that lets you move through and select text with your keyboard. F7 toggles it, so if you ever want to use this feature you can press F7 to turn it on, then again to turn it off.
(It is most likely that you had pressed F7 by accident to turn it on this time.)
Another way to turn caret browsing on and off is to go to Preferences > Advanced and check/uncheck the box labeled Always use the cursor keys to navigate within pages (under Accessibility).
But F7 is easier.