Bad request errors are usually about corrupted cookies.

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache":

• Firefox/Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Firefox/Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies"

If clearing cookies didn't help then It is possible that the cookies.sqlite file in the profile folder that stores the cookies got corrupted.

• rename/remove cookies.sqlite (cookies.sqlite.old) and if present delete cookies.sqlite-shm and cookies.sqlite-wal in the Firefox profile folder in case cookies.sqlite got corrupted.
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Hey cor-el..

I appreciate the suggestions but unfortunately they did not fix the problem. I just cleared all the cookies out from the preferences. Same problem. I did notice that a cookie for the modem was left, but closing the browser deletes it. So it is a fresh slate in terms of cookies for these modems whenever I start the browser.

I also found and deleted the cookes.sqlite file. Again no change in the situation.

Playing around here tonight, I learned a few things and have information to add.

This post hopefully has an image attached. It is a snapshot of my address bar when the error occurs. I changed the IP address to something meaningless. But I think there is a problem with the stuff after the ?

I can login successfully with Internet Explorer. After doing so, I copied everything from the address bar, including all the parameters after the question mark. When I pasted that into Firefox 53, it worked fine. I can do what I need to do.

But I don't want to jump through that hoop. I'd much prefer a fix or a more convenient work around. Any ideas anyone?

Thanks!

This looks that you try to access your router.

Are you using a bookmark or is this happening when you click a link of the router's main page (192.168.1.50:9191)?

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache":

• Firefox/Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Firefox/Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies"

That's right. This does look like trying to access my router. It is the same idea, only it is a cellular modem.

The problem is not cookies or the cache. I tried those things that you suggested.

At the login page, I opened the Web Console. There is warning message saying that the character encoding of the HTML document was not declared. This, I believe, is the root of the problem. Unfortunately, these devices are not particularly new and getting the manufacturer to declare an encoding may not be possible. But I intend to try through their support website.

I think that prior to Firefox 53, Firefox managed to handle the situation. But something obviously changed, and Firefox does not handle the characters that come back as it used to.

If there was some way to set a default encoding for documents that lack a declaration, that may help. Know of any way to do that?