This conclusion is only justifiable with the assumption that the salience of politics to the community with which the incumbent in the office in question must deal is fixed. This is a fairly counterintuitive claim that requires some justification; salience of issues -- particularly those aside from the ones that define a particular community -- is generally quite fluid.
Do you agree that if he'd never made the donation he'd still be there?
From the very limited information I have, my impression is that he was a poor choice with weak support from the board from the outset and most likely a deficit in either ability or willingess to manage community relations rather than technical issues that would quite likely not have lasted long or been particularly effective as CEO at Mozilla at the time (and with the board in place at that time) he was appointed even if that particular source of controversy hadn't erupted.
Would he still be there now? Maybe. Hard to say.
No, I don't think that's a justifiable claim. A political donation was the trigger for a public controversy, which was a key event leading to the previous CEO leaving the position. The evidence does not, however, provide anything like clear support that the political donation itself was the sole reason that the CEO left the position.