static/landingpage: show warning box for firefox users about issue #2875
static/landingpage: show warning box for firefox users about issue #2875

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