Could you copy/paste any additional wording from the error page? For example, does it mention a secure connection problem or is it a regular HTTP address?

Does the device use a framed page? To check that, right-click the error and check the context menu for a "This Frame" item. If you find that, try expanding the This Frame menu and using Open Frame in New Tab. Any difference in behavior?

The error message is as follows:

Unable to connect securely

Firefox cannot guarantee the safety of your data on xx.x.xx.x because it uses SSLv3, a broken security protocol.

Advanced info: ssl_error_unsupported_version

Unfortunately, Firefox 39 and later do not support the old SSLv3 protocol and some routers are not able to use newer protocols.

For the time being, you could use Firefox ESR, the extended support release, which can still support SSLv3 via some preference changes, I believe.

To install ESR, I suggest this to avoid folder confusion:

(A) Download an installer for Firefox 38.5.2esr from https://www.mozilla.org/firefox/organizations/all/ to a convenient location. (Scroll down to your preferred language.)

(B) Exit out of Firefox (if applicable).

(C) Rename the program folder, either:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\Fx43

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\Fx43

(D) Run the installer you downloaded in (A). It should automatically connect to your existing settings.

Next, in order to allow Firefox 38 to use SSLv3, I believe you need to do this:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste TLS and pause while the list is filtered

(3) Double-click the security.tls.insecure_fallback_hosts preference and enter that hostname or IP address

You may need to exit Firefox and restart it for that to take effect. Does it work?

Thanks for your help.