Firminger, regional sales director for Northern Europe and the face of Yahoo's search operation in the UK, has left without a job to go to and is thought to be taking time out before re-entering the online sector.
His departure further dents Yahoo's senior UK executive tier and is a blow to the company as it seeks to close Google's dominance of the UK search market.
Three senior sales executives left for online rival Facebook in October and November last year, with commercial director Blake Chandlee the first to depart, followed by head of agency strategy Stephen Haines and group head Jon Harvey.
As of December, Google commanded 81% of UK search spend, while Yahoo took just 6%.
Since Chandlee's departure, Yahoo has merged its display and search teams and has created an as-yet unfilled UK and Ireland commercial director role spanning both disciplines. Previously, Chandlee headed display and Firminger led search.
It is understood that Firminger's departure was amicable and not part of Yahoo's reported plan to cut 1,000 jobs worldwide. He declined to comment.
Yahoo declined to comment, other than to say it was not ready to announce any new appointments.
Firminger spent four years at Yahoo and was the architect of the roll-out of Panama, Yahoo's relaunched paid search platform, which went live in the UK last year.
Agencies reported a vast improvement on the previous model and a largely smooth transition period, but Yahoo, like Microsoft, has failed to erode search behemoth Google's dominant market position in the UK.
Research firm comScore last week reported that both Microsoft and Yahoo posted a fall in their number of UK search queries in December, recording their lowest figures since such records began.
Prior to working at Yahoo, Firminger was UK managing director of Enpocket, the mobile marketing agency bought by mobile phone giant Nokia last year for a sum thought to be in the region of £100m.
His departure comes amid speculation about the future of Yahoo. It has rejected Microsoft's approach and is reported to be in merger talks with AOL.
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