Unfortunately, there is no way to remove or re-assign built-in keyboard shortcuts. However, sites can intercept most of them and replace their behavior.

Did the site previously intercept F3 successfully, but it stopped working at some point? This could be the issue:

Firefox 65 changed how key events on non-printing keys are sent to websites. Previously, a site would get a "keydown" and a "keypress" for F3, but starting in Firefox 65, it only got a "keydown". If the site's scripts rely on getting a "keypress" then it lost its ability to detect that you pressed F3 and won't be able to re-purpose that key.

If you think that might be the case, you could try the following workaround.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button to accept the risk.

(2) In the search box above the list, type or paste hack and pause while the list is filtered

(3) Double-click the dom.keyboardevent.keypress.hack.dispatch_non_printable_keys preference to display a dialog where you can list sites that should have the old behavior and add the host name, then click OK

Mine shows this by default:

www.icloud.com

Let's say for example that you have a problem with

https://support.mozilla.org/en-US/questions/1261273

The hostname is the part between // and / in that example, which I have marked in bold. So you would edit the preference value to:

www.icloud.com,support.mozilla..org

and then click OK. (Separate the host names with commas but no spaces.)

If you try that, you will need to reload the site (Ctrl+r) or load a different page before it takes effect. Does it make any difference?

jscher2000, I followed your instructions and it worked like a charm! I can't thank you enough!