hello missdeezy, when i try this the page is scrolling further automatically when i get towards/beyond the bottom of the page with the mouse pointer - alternatively scrolling with the mousewheel, the arrow-key & the page-up/down-key works during text selection.

in case this doesn't work for you, you may try to launch firefox in safe mode and see if you can replicate this behaviour there? if not, maybe an addon is interfering...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

I am able to highlight and scroll down by dragging the cursor below the bottom of the Firefox content window. The further the cursor is below the page content, the faster it scrolls and continues to highlight. I don't find it necessary to do "finger gymnastics" with the scroll wheel, the down arrow key, or the Page Down key.

Does it help if you enable the Add-ons bar (Ctrl + /) or the find bar (Ctrl + F)?