Tell them this isn't personal but corporate.
From what I can find out Mozilla gets like 70% of it's funding from people using Firefox's search bar. So just don't use that. Or better yet replace Firefox with another browser like Pale Moon.
Never used FF’s search bar. I use DDG or IxQuick. FF is the best browser, IMO, and I use Thunderbird for mail. Cutting out Mozilla is not the answer, IMO. I just refuse to funnel money to them through search or other means.