If you use Page Up and Page Down to navigate longer web pages in Firefox, but it’s suddenly stopped working, or seems to be working on some pages but not others, you’ve probably turned on Caret Browsing. Try hitting F7 and see if that fixes it.
Caret browsing can be useful in some contexts, and is an accessibility feature aimed at people who may find it difficult or impossible to use a mouse.
Caret and stick
Somewhat surprisingly for an accessibility feature, the caret itself can be quite hard to see. Worse, it doesn’t always appear anywhere you can see on a newly-loaded page, as described by a Mozilla Support forums user back in 2013:
1) Switching to a page when already in caret mode requires using the mouse to set focus somewhere before caret navigation makes sense. I have tried work arounds using text searching or tabbing but this is both unrelible and unnatural (even tedious).
2) While in caret mode I sometime loose site of the cursor. I can keep using the arrow keys until the page starts to scroll which tells be that the caret is on the top or bottom edge, but I still can’t see it.
3) While scrolling with the arrow keys, the cursor/caret hits a link or heading then further use of the arrow keys increments lines at that “indentation” level, bypassing whole paragraphs of text. I need to use the mouse to reset the caret an area of text.“Problems with caret browsing”, @arbalest, Mozilla Support forums
There are more complaints, but these first three really chime with me. A couple of days after their initial post, @arbalest resolves their own query by noting that at least some of the the strange behaviour of Caret Browsing explained by a bug in Mozilla:
Caret browsing is not available if there is no selection rangesMozilla Bug 144000
This is the new bug, just like the old bug
Incredibly, this bug was first filed over 20 years ago on 2002-05-12. Even more incredibly, it still has tracking status NEW, which means “This bug has recently been added to the assignee’s list of bugs and must be processed.”
I’m not convinced that an issue which is roughly the same age as Nickelback’s “How You Remind Me” really counts as new or recent, but I digress.
A technical discussion of how best to address the issue goes on for around a decade, then dries up in 2011.
Who is this bug assigned to? No-one, unfortunately. Although Hsin-Yi Tsai, a Mozilla Senior Runtime Engineering Manager, mentions that this ancient bug was reviewed as recently as in April 2022, we don’t know what the nature or the outcome of that review was.
Hard to avoid the conclusion that this bug is destined to outlive us all
and eventually cross the heliopause to drift forever through the vast majesty of interstellar space.