I see no errors/gaps. Sorry.

You have not ticked send info to Mozilla so I have nothing on your cpu/system to allow me to help in depth. Next time before submitting a question please tick the box so we can help you better.

Current version of Firefox is 56.0.2 https://www.mozilla.org/firefox/all/ Firefox 57 Quantum release date Nov 14th https://www.mozilla.org/en-US/firefox/quantum/ Note all Legacy Extensions will be disabled or removed.

Since do not know what Extensions you are running please : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode and https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration TRY NO HARDWARE ACCELERATION

Try using Firefox Reader view.

Please let us know if this solved your issue or if need further assistance.

So you have bad sight? Here is a zoomed version for clarity: https://i.imgur.com/K6lIZJj.png

That is caused by the website using an increased line-height setting (1.6) that causes the gaps when you select the text. You can check that in the Inspector.

You can right-click and select "Inspect Element" to open the builtin Inspector with this element selected.

• https://developer.mozilla.org/en/Tools/Page_Inspector

That's dumb. Please add a setting to Firefox to ignore any line height settings, just like there is an option to ignore any color. Even worse, I found justification in one site, when the correct thing to do is never use justification on monospaced fonts.

Also, consider looking at this even worse problem: https://support.mozilla.org/en-US/questions/1174466