You can already add a Safe as an account in Rabby and there might be similar integrations with Metamask and others in the future. But I agree, there might also be value of having a dedicated browser extension for Safe, optimising the UX around Safe accounts.
I am personally not a big fan of browser extensions (compatibility issues with different browsers and other UX and security concerns). But there is probably a sizeable amount of users that would like to use something like this. At this point there are no concrete plans from the Safe core team to develop a browser extension, but I’d love to see another team seize that opportunity.