Eich's donation was well-known. The lists are public. Lots of Mozilla folks were aware and unimpressed, but they tolerated it because Eich (CTO) was a respected member of the corp with a long history of technical excellence. As well as no workplace hostility etc.
Then Eich was to be promoted to CEO, which is a nontechnical job that involves being the public face of the corporation and speaking on behalf of all employees.
A substantial portion of Mozilla employees felt that this new expanded role would not be appropriate for Eich or anyone who had expressed hostility to a marginalized group, that Eich did not represent the values of Mozilla, and that they did not want to promote him or to be led by him.
So they created an internal stir which gathered a lot of internal support, bled into public forums, and led Eich to back out of the CEO role. He probably could have gone back to CTO, but he decided to resign from the organization instead.