Highlights the wrong words with non-standard font size

[Lachlan-Gardner]

Describe the bug
This is for the Firefox extension (is this the right place for it?). The spellcheck highlight boxes are completely off when using something besides 100% scaling for font size.

To Reproduce
Steps to reproduce the behaviour:

1. Go to Firefox settings, then accessibility.
2. Change font size to 140%. (Edit for clarification: You change the default zoom, then tick font only.)
3. Open a new tab and write somewhere, for the best effect, more than one line.
4. See error

Expected behaviour
The highlights to be over only the correct (or I guess incorrect) word or section.

Screenshots

Platform
Firefox extension.