You need to look at history. In early 90s why did Microsoft invest in apple when it is its competitors. Investment doesn't mean they are medling into mozilla business. For companies like google (present) or Microsoft in 90's. It is better to have a crippled competitor than no competitor. No competitor attracts government agencies for monopoly which is worse.

In the 1990s Microsoft “invested” in Apple because Steve Jobs allowed them to save face by giving them the option to settle their part of Apple v San Francisco Canyon Co by calling part of it—$150 M—a stock purchase that only lasted a few years. I do not know how much the total cash settlement from Microsoft was, but industry rumors went up to $1B.