Well, this is odd.

Yahoo is reportedly displaying search results – both organic and paid – that are powered by Google when some users search using Mozilla’s Firefox web browser. That is according to Aaron Wall at SEOBook, who has screenshots comparing the usual Bing-powered version of results and the Google-powered ones.

It’s completely unclear just how widespread this is occurring, whether it’s just a test, or whether this is something more users can expect to see.

According to Wall, it’s only happening in some versions of Firefox. I’ve tested the most recent update to the browser on Mac, as well as Chrome and Safari, and have been unable to produce a Google–powered search results page by performing a Yahoo search. It’s all Bing for me so far. For the record, I did this using the “seo tools” query that Wall displays in his screenshots.

It’s been an interesting period for search engines powering other services. There have been a lot of changes in the past year, and things are starting to get a bit convoluted.

As you probably know, Yahoo and Microsoft have had a search and advertising partnership in place for many years. This came about after a Yahoo Google deal fell apart over antitrust concerns. Over the years, it became apparent that Yahoo wasn’t incredibly happy with the arrangement – particularly since ex-Googler Marissa Mayer took over as CEO.

Earlier this year, however, the two companies amended their agreement. While some expected them to part ways, the companies decided to stay together, but under terms that are largely better for Yahoo, which gets increased flexibility to enhance its own search experience on any platform. The partnership is non-exclusive for both desktop and mobile. Yahoo will continue to serve Bing ads and search results for “most” (51%) of its desktop search traffic, it said, and can do whatever it wants on mobile.

It can also do whatever it wants with that other 49% of desktop, which is presumably where these Google-powered results come in. We haven’t heard anything about any kind of arrangement between Yahoo and Google (who are already battling for the loyalty of Firefox users), but that doesn’t mean there isn’t an arrangement.

In fact, the companies have expressed interest in working with one another even since Mayer has been running Yahoo. We’ve reached out to both companies for comment, and will update accordingly.

Late last year, Mozilla and Yahoo announced a partnership that sees Yahoo take over the default search experience in Firefox in the U.S. – a spot that used to belong to Google. Since then, both Google and Yahoo have been employing various tactics to get users to select their respective search engines as their defaults.

In the most aggressive instance of this yet, Yahoo entered a partnership with Oracle to prompt those downloading Java updates on Chrome and Internet Explorer to switch to Yahoo. This is a big deal considering that Java is the most popular programming language and Java software is reportedly installed on 89% of desktop computers.

Making the search landscape even more complicated, this week also saw the announcement of a new long-term deal between Microsoft and AOL, which sees Bing taking over the Google-powered spot for AOL, which is now owned by Verizon.

Update: Search Engine Land got a statement from Yahoo: “As we work to create the absolute best experiences for Yahoo users, from time to time, we run small tests with a variety of partners including search providers. There is nothing further to share at this time.”