Unfortunately I had the same issue on my Android phone - every time I re-opened the app I had to open a new tab in order to browse. I ended up installing and using Firefox Nightly instead (where the issue seems to be resolved).
Have you tried again since 57 was released? Or tried nightly? I've been using Firefox on mobile for a long time, and I remember experiencing similar issues from time to time, but I haven't had these in a while.
FWIW, closing tabs once in a while can also help. Mobiles typically don't have too much RAM.
Had a similar issue recently. Reinstalling the app seems to have fixed it.