Perhaps an underlying issue is your Mac build model year
reached the 'end-of-the upgrades' road. Includes Firefox ESR,
goes no further, without update steps for El Capitan 10.11.6.
Other browsers which may still be supported by their makers
include [ Waterfox ] or [ Brave ] (links) & perhaps others.
If your Mac were built in 2012/2013, it'd see further upgrades.
There had been an issue with 'secure certificates expiry'
one work around existed, for older macOS & El Capitan:
• Q: "Many websites not loading in El Capitan.." - Apple Community (Oct 2021)
https://discussions.apple.com/thread/253272998?answerId=256123246022#256123246022
The "Let's Encrypt" site has some ideas [may help FFox]
for awhile. ~A newer browser such as Brave or Waterfox
could work, by introducing all newer certificates.
