Firefox text search - How to disable automatic wrap-around when reaching the end of a document

I believe that this is impossible.

The bug-report Bring back option for wrap/no wrap when using find toolbar was opened 16 years ago. Its status is "RESOLVED WONTFIX" since 16 years ago. Users kept hammering on this decision for 8 more years, without any result.

The person that closed the bug-report, Mike Connor, Mozilla Director and past Firefox Architect, did write this:

There's no point in adding a preference for the sake of "user choice." In fact, there is a much greater benefit in reducing the degree of user choice and configuration required, and focusing on creating an optimal experience without configuration.

Unless you can come up with a use-case where this is actually useful/needed, that's the way it will remain.

Later on he added:

No, the method of enabling it isn't the issue. Its the UI itself once enabled that I'm talking about.

Wrap by default and without interruption is the choice we've made. We're not going to change core behaviour like this. We don't want dialogs of any sort, so those suggestions are out the window.

If someone really wants to implement a different find mechanism, then that's what extensions are for.

And more:

We chose a different method, one that has found a great deal of user acceptance. Like any decision we've made from the outset, there will always be someone who wants a different way of doing things. Instead of maintaining multiple codepaths and multiple prefs, we've elected to leave alternate ideas like this to extensions. You might not agree with how we do things, but that's been the driving force behind the project.

I have not found any add-on for disabling search wrap.

My answer is then negative - this is impossible to do. I don't know if it's possible to do so by add-on, but nobody has written one till now.