What kind of "old gear"? Routers or hardware firewalls?

As far as Firefox goes, you seem to have a number of questionable extensions installed.

First, disable that HTTPS Everywhere extension and see if that helps.

2nd - how about Ghostery? https://addons.mozilla.org/en-US/firefox/addon/ghostery/reviews/ A lot of poor reviews lately.

3rd - IE Tab 2 (FF 3.6+) 6.2.18.1 https://addons.mozilla.org/en-US/firefox/addon/ie-tab-2-ff-36/reviews/ Also a lot of poor reviews lately.

It is possible that this old equipment only supports weak RC4 cipher suites. Support for RC4 has been deprecated for a long time and has finally be removed in Firefox 50+.

• https://www.fxsitecompat.com/en-CA/docs/2016/rc4-support-has-been-completely-removed/

You can consider to install the portable Firefox 45.6.0 esr version to access websites that do not work with the current Firefox release. The portable version comes with its own profile folder and doesn't interfere with currently installed Firefox versions.

• http://portableapps.com/apps/internet/firefox_portable
• http://portableapps.com/apps/internet/firefox-portable-esr

• http://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%20Portable%20Ed./

• http://www.freesmug.org/portableapps:firefox#toc1

• http://portableapps.com/support/firefox_portable#helper_apps

cor-el said

You can consider to install the portable Firefox 45.6.0 esr version to access websites that do not work with the current Firefox release.

Note you will need to toggle security.tls.unrestricted_rc4_fallback preference to false in about:config in the portable Firefox 45 ESR in order to load RC4.

It would be best to add the website to the whitelist and leave security.tls.unrestricted_rc4_fallback at its default.

You can open the about:config page via the location/address bar and use its search bar to locate this pref:

• security.tls.insecure_fallback_hosts

You can double-click the line to modify the pref and add the full domain () to the value of this pref. If there are already websites (domains) in this list then add a comma and the new domain (no spaces). There should only be domains separated by a comma in the Value column (example.com,www.example.com).

• Bug 1138882 - Create a separate a pref to enable unrestricted RC4 fallback

Well either way as otherwise portable Firefox 45.6.0 ESR will work the same as Firefox 50.0 and later when it comes to RC4.