Whenever I type something into a text field (address bar, form field etc.) that uses the system default font (DejaVu Sans size 10) and press the "left arrow" to move the caret backwards, the caret leaves artifacts / turds in the text field. It does happen with other fonts as well but it only happens in FireFox. Using a font that causes problems in FireFox in another program does not yield the issue so it is related to FireFox. I've tried to create a new profile (in Linux) to rule out any settings / add-on issue.

System info: Core i3 6100, Asus B150M, 16GB DDR4, OnCPU GFX, Linux Mint KDE 64bit, FireFox V47.