As described here RStudio has ligature support. I currently use version 1.2.1335 and cannot find this checkbox. It is necessary for me, because my current font has ligatures and when RStudio shows one, like the one for fi, the position of the cursor is not accurate any more and causes problems when editing a line after a ligature.
That checkbox is no longer present in RStudio 1.2.1335 but ligature fonts work normally, For example I also use Windows 10 and firecode font works as expected for me, maybe your issue is with the specific font you are using.
My company uses the font "Attribute Mono" (https://www.fonts.com/font/fontfont/ff-attribute-mono). It has a ligature for 'fi' which is shown in RStudio but afterwards the cursor is on the wrong position. If you want to delete a character it is always one before the expected one.