Ubuntu 17.10, changing input switching to alt-shift (with gnome-tweaks) creates problem in firefox
Use shift+alt (i.e. shift before alt) in Firefox.
I have the same issue with slack, after some digging found out that there is another way to change the keyboard layout w/o using gnome-tweak-tool which wont cause this side effect of losing the focus.
The solution is to use gsettings as explained here
gsettings set org.gnome.desktop.wm.keybindings switch-input-source "['<Shift>Alt_L']"
gsettings set org.gnome.desktop.wm.keybindings switch-input-source-backward "['<Alt>Shift_L']"