I have looked into it and consider this a bug in Firefox. Apparently, it inserts space between <mi> elements when at least one of them has an explicit mathvariant (and in particular, even when both have the SAME math variant). That is what is happening here. TeX considers each character in {\rm Opname} to be a separate variable, and puts it in its own <mi>. While you know that all six characters act as one unit, TeX doesn't.
In general, I'm not planning to try to work around bugs (or in this case, misfeatures) in the various MathML implementations. You may wish to report this problem to the Firefox development team.
Davide
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
I have looked into it and consider this a bug in Firefox. Apparently, it inserts space between <mi> elements when at least one of them has an explicit mathvariant (and in particular, even when both have the SAME math variant). That is what is happening here. TeX considers each character in {\rm Opname} to be a separate variable, and puts it in its own <mi>. While you know that all six characters act as one unit, TeX doesn't.
In general, I'm not planning to try to work around bugs (or in this case, misfeatures) in the various MathML implementations. You may wish to report this problem to the Firefox development team.
Davide