Fix backup download for Firefox.
Fix backup download for Firefox.

decaffeinate-examples/vimium,poacher2k/vimium,mrmr1993/vimium,smblott-github/vimium,decaffeinate-examples/vimium,poacher2k/vimium,philc/vimium,mrmr1993/vimium,gdh1995/vimium,smblott-github/vimium,philc/vimium,philc/vimium,gdh1995/vimium,mrmr1993/vimium,poacher2k/vimium,decaffeinate-examples/vimium,gdh1995/vimium,smblott-github/vimium