Thanks for the report. We're aware of this problem, and working on fixing it: https://bugzilla.mozilla.org/show_bug.cgi?id=617298

Until we (or Swiftkey) manage to fix this problem, you can work around it by using a different keyboard with Firefox.

A fix for this bug will be included in the next version of Firefox, which will be available in a month or two. You can help us test the fix now by downloading a "Nightly" or "Aurora" preview build from this page: https://wiki.mozilla.org/Mobile/Platforms/Android