I'm interested in hearing about both the options available on different operating systems and/or mobile devices and the primary benefit each provides.
On Android, Firefox Focus has become my normal browser. I really don't use the phone to browse all that much, and prefer to use a clean session each time.
On Windows, I use multiple off-label browsers[0] mostly just to compartmentalize different sessions simultaneously. Now that Firefox is headed towards supporting this as a first-class feature, I may be able to cut down to one again. I just can't bring myself to give Google/Chrome all of my browsing metadata, but I do use it for work-related access since GMail is tied in.
[0] - Qupzilla, with separate-per-window sessions in private browsing mode - no extra work from me; also K-Meleon
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Edit: Thanks for all the testimonials; I will definitely be checking these out!