Add Wall of Browser bugs entry for #793

Also, link to the Mozilla bug in the relevant callout.
Merge pull request #14527 from twbs/firefox-disabled-bug

Add Wall of Browser bugs entry for #793
add entry for #14528 to Wall of Browser Bugs
Merge pull request #14531 from twbs/doc-14528

add entry for #14528 to Wall of Browser Bugs
Replace nonexistent .col-6 classes in offcanvas example with .col-xs-6

Fixes #14500
Merge branch 'thr3-arendt/patch-1'