I get this too, my firefox is always switching me from en-GB to en-US.
Application Settings
Requested Locales ["en-US"]
Available Locales ["ka","gu-IN","tl","hr","ff","gl","et","it","sl","cak","nn-NO","son","hi-IN","pa-IN","eu","az","lt","en-US","ca-valencia","gn","be","kab","lij","nb-NO","ne-NP","sv-SE","es-MX","zh-CN","dsb","fi","ach","lv","my","br","vi","pl","ms","en-GB","rm","ja","uz","ar","hsb","sr","th","gd","ast","ta","ur","trs","te","hy-AM","mr","pt-BR","uk","eo","nl","mk","ru","bs","af","ga-IE","da","pt-PT","cs","fr","kk","bn","el","fa","he","kn","an","bg","km","sk","fy-NL","si","ko","de","sq","xh","zh-TW","cy","id","es-CL","is","es-AR","ro","ca","hu","oc","en-CA","ia","tr","es-ES"]
App Locales ["en-US","en-GB","en-CA"]
Regional Preferences ["en-GB"]
Default Locale "en-US"
Operating System
System Locales ["en-GB"]
Regional Preferences ["en-GB"]