You’re right on the numbers....Firefox never had majority share. The stronger claim is causal influence, not dominance. I recently read somewhere that the Firefox (and later Chrome) forced standards compliance and broke IE’s de-facto monopoly mindset. IE’s decline was gradual and multi-factor, but Firefox clearly shifted developer and user expectations.
No one is claiming, here or in the article, that Firefox ever had a majority share.
I don’t know if the 55% number for IE is 100% correct but it sounds like the right ballpark to me. The browser market was a lot more fragmented 15+ years ago, so saying that IE had 55% market share and Firefox had 32%, leaving 13% for other browsers, sounds completely right to me.