fix https://github.com/sagemathinc/smc/issues/1935 -- "keyboard shortcuts firefox"
fix https://github.com/sagemathinc/smc/issues/1935 -- "keyboard shortcuts firefox"

tscholl2/smc,tscholl2/smc,tscholl2/smc,sagemathinc/smc,sagemathinc/smc,sagemathinc/smc,DrXyzzy/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,DrXyzzy/smc