Currently, Debian Testing is "frozen". This means no new upstream versions will enter it. This creates a tailback that ripples into Unstable. Not all bugs are security-relevant. Those that are, are patched by the Debian maintainers into the versions in Stable. For those types of software, where patching is not possible or not practical, there is the Debian security repository. Firefox and Chromium reside here for example. This approach however is time-consuming, so it's not done for a lot of packages. "Minor" browsers don't get that luxury. If you build your own packages from newer upstream sources on top of Debian Stable's well hung package base, then you'll have to make sure on your own that there are no ABI breaks. This is why I wouldn't want to build my own newer browser packages and why I wouldn't want to have the responsibility of doing it for others.