This reaction baffles me and feels like online indignation culture. All I could see, as a Latin American English speaker, is someone who's sincerely trying to help and picked their words carefully. Your proposed phrasing sounds unnatural and AI-like.
You're touching the heart of the question, but not in the way you think. Any arbitrary statement could be construed as offensive by SOME culture out there. But people prefer to immediately assume the worst and be offended instead of giving others the benefit of the doubt.
For instance, if I followed the principles of indignation culture, I could be offended by your "I can assure you about that :)" statement. I could say "Are you making fun of me?!?!"
To be clear, your statement is perfectly fine. I understand what you're saying, despite disagreeing.
Also, this happened in an English language forum. It's reasonable to expect that foreign speakers would be somewhat versed in the conventions of the English language and see that the comment was not offensive at all.
It's not just the words that differ. Rudeness can come from underlying values betrayed by language, and those values do matter -- words aren't simply interchanged atop them.
Some cultures value relational power in leadership, and so their language reflects preserving relationships as a base resources. Some cultures value authority, and so their idea of being polite might involve some maintenance of authority at the expense of perceived care or relationship.
I'm just saying the words that convey rudeness are not necessarily just a superficial dressing on the same thing. Politeness is a shorthand that is also about alignment with a cultural value. Cultural values differ. My being annoyed at someone else's cultural posturing as default, that's not just mindless indignation. Values matter.
As a native speaker (American) the phrasing is classic condescending soulless corporate customer service speak. 1) You must always apologize, 2) you must never admit fault. "I'm sorry you feel this way about what we did" comes across _to me_ as "what we did was totally fine, it's too bad that you don't understand the wisdom of our actions." That kind of phrasing is also a bit of a trigger because the majority of the time you hear it from companies that don't give a damn how you feel and will fight to avoid doing anything to actually help you.
It's of course impossible to say if this was just an unfortunate choice of phrasing or if it's a sign that Mozilla has become that soulless corporate entity (I say this as a Firefox user for more than 20 years).
