Why is the font spacing on some websites broken?

Solution 1:

it was "Helvetica" for me. Had to install for some Scribus compatibility and since started noticing fonts overlapping everywhere - documents, web pages, web-based editors. Removing the font fixed the issue.

Yet to figure out why the incompatibility but as far as this issue is concerned, look for that font or any other recently installed fonts.

Solution 2:

In Firefox, check your zoom setting and character encoding. Reset the former and the usual default for the latter is UTF-8.

Solution 3:

Finally! i fixed it.

there where some botched fonts installend in my /home/.fonts/ directory.

I deleted them now everything is fine.