negative letter-spacing value breaks tab indentiation in firefox

On Firefox 53.0.2 (32bit) indents are broken when viewing code.

This is the case when the code you view uses tabs as indents.

If space is used the indents are correct.

You can have a look at an example.

https://bitbucket.org/frankmohaupt/public/src/111532a66d9d76ab8f0b06699fbcf5b99fb7a5ec/letter-spacing-issue.php?fileviewer=file-view-default

NOTE: Use Firefox to see the issue.

This issue is caused by a negative letter-spacing css setting:

#!css

letter-spacing: -0.005em;