I looked at the carousel images at mozilla.org and skimmed the article about anonymity to try to decipher what you mean, but I'm not sure what I'm looking for. Could you be more specific – what is the "virtue signal", and why is it so problematic?
However, to be cheeky, if you wanted to talk solely based on which photos passed editorial approval, compare those represented to the physical attributes of their leadership team: https://blog.mozilla.org/press/media-library/bios/
As other people mentioned, it's an overused expression, but it's meant to call attention to an outward expression of a particular viewpoint or ideology, particularly with the aim of increasing the standing of an entity within a particular group.
> why is it so problematic?
Where it becomes "problematic" is when the need to communicate cohesion with a cause does so at the expense of reality or otherwise weakens a campaign. Whereas before I stood in unwavering support of Mozilla, their language now seeks to define their company as the crusader against the "others" who would seek a less "inclusive" or "respectful" internet, and coupled with their imagery, they, as a company, are communicating identity politics in those that they feel are underrepresented or deserve special attention based on physical attributes -- which goes counter to the "hacker" mentality against genetic fallacies/the ideal that an idea is more important than the physical attributes of the person that expressed it.
LMGTFY https://www.google.com/search?q=virtue+signalling https://en.wikipedia.org/wiki/Virtue_signalling
Virtual signaling is a public demonstration of values, but usually with the negative connotation of an ulterior motive to improve social standing and collect praise.
"Wearing your religion on your sleeve" is an older, similar expression.
I don't think this applies here.
It can also apply to anything that a culture considers virtuous or trendy, though.