I am on 20.04, and have only Firefox installed: I could experiment only with that, and not with Chrome.
I could reproduce your issue: when I right-click anywhere on Firefox's window or on the top window decoration, the "Always on top" option is missing, along with other common window-manager related options.
Yet, they can be accessed; the only condition is that the window needs to be in non-maximized state.
When one moves the mouse cursor along the edges of a (not maximized) window, the cursor changes to indicate that a click-drag along that edge would start resizing the window.
When the cursor is in this resizing mode, a right-click reveals the original window-manager context menu, along with the "Always on top" option, which even works as intended.
The size of the drag-to-resize area is configurable:
gsettings set org.gnome.mutter draggable-border-width 20
This will give you a 20px range along the window edges to expose the desired resizing mode.