The above boxes have padding-left:10em, and different specified font-families. Firefox and Safari have usually a default font size for monospaced fonts smaller than for the other font-families (13px versus 16px, at least in a "common" configuration.) This is shown by a different left padding in the third (at least) of the above test cases. I guess this is an attempt to counteract the fact that some common monospaced fonts appear bigger (at the same font-size) than proportional fonts. But I don't like this type of solution which, among other problems, makes monospaced text size much different amongst browsers, at least in default conditions. Moreover the default 'compensation' (13/16) appears excessive.
The smaller size seems to apply:
If a monospaced font is specified, but without the "monospace" generic family (not a good idea, but possible), then the different size is not applied, in both Safari and Firefox. A better 'solution' (see the first two references below) is using the "monospace" generic-family, but adding after it a further generic "serif" or something like that: this avoids the size problem, and still provides a monospace generic family if the previously mentioned ones are not available.
References: