This is already fixed in the current version of Firefox which will be available as a security update before Ubuntu 17.10 is released.
There are a few workarounds for now to get the current version of Firefox in Ubuntu 17.10 if you really need it.
One is to manually enable the zesty-security archive.
Another is to use the firefox-next PPA to try out the next (beta) version of Firefox.
This is already fixed in the current version of Firefox which will be available as a security update before Ubuntu 17.10 is released.
There are a few workarounds for now to get the current version of Firefox in Ubuntu 17.10 if you really need it.
One is to manually enable the zesty-security archive.
Another is to use the firefox-next PPA to try out the next (beta) version of Firefox.