The storage backing for Application Cache has been completely disabled
starting with Firefox 84 . That means the current
window.applicationCache object is not really useful and only exists
for backward compatibility. The plan is to remove it.
We intend to do this in two stages: First we disable the API for
Nightly and Early beta  followed by an eventual disabling and
removal in Release . The removal in Release is expected to happen
after Chrome's origin trial is completed .
The risk associated with removing this API is slightly higher than
with disabling the storage from appCache due to the greater chance it
could cause JS exceptions to be thrown if websites don't check if the
However, we have successfully unshipped this from insecure origins in
the past so we don't expect major issues.
The current use stats for the window.applicationCache in Beta 86
(2021/01/25 - 2021/02/17) are as follows:
018.91B (99.51%) - Not used
1244.32M (0.49%) - Used