Recently I’ve had a serious sounding offer to inject an ad, i.e. a one-off ad would open in a new tab when the extension updates, for $3K a pop, which I just ignored, then he emailed again saying $4K, then just yesterday he emailed again with a bunch of emoji and said what about $8K.
It’s tempting, but it would still be selling out my users, who may be ungrateful little brats but I could never do that to them, I value their approval too much.
The dev for uBlock origin must have received million and maybe tens of millions dollar offers, yet they refuse so much as token donations.
What is the ad for? If it is a US equivalent to Great Ormand Street Hospital or some other worthy thing, then why not! I suspect it isn't and you will be offered quite a lot more vapid dollars because your user demographic is ... nerdy and installs addons 8) That is worth a lot more than 0.4c per head.
It may be that the ad offers are not as unpleasant as we might make them out to be but you do need to live - up to you. However I suspect they are just as genuine as the crap that lands in my Inbox, sometimes.
I recommend not describing your users as brats - https://en.wikipedia.org/wiki/Gerald_Ratner