So, if you rushed to install Windows 10, and Mozilla's latest Firefox 40 was pushed to your computer (by default), and you happen to live in a country that Cortana understands you... Firefox 40 will automatically re-route queries that Cortana sends to Microsoft's Bing to whatever your default search engine is.
Thanks god, this default setting can be opt-out.
source: Firefox hijacks Cortana searches on Windows 10 (Geek.com, 12 Aug 2015)
Is this a big deal? Probably not, but it was known that browser company and search engines hadcommercial agreements that browser makers earn commissions by sending search queries through built-in search bar. (At least this was the case in the past.)
UserAgent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/44.0.2403.107 Safari/537.36 OPR/31.0.1889.99 (Edition Campaign 67)