In the past few years I've sometimes ran into websites that don't work in Firefox on Linux, and I'm trying to understand why so I can notify the owners with more than just a vague “it doesn't work”.
Now this happens of course. While most web developers do test in Firefox, not many will have tested their products in Firefox on Linux, and some really don't care. Some only target Chrome/Webkit and don't bother with Firefox at all. That is not what this question is about though.
There is something here that makes me suspect that there is an underlying cause that is repeated on seemingly unrelated websites, and I suspect some repeated bit of configuration code or web content serving library or application that does this. Something is fishy.
The problem
The websites affected return only a plain HTML message with a 403 HTTP status code for any resource requested; it looks like this:
Forbidden
You don't have permission to access / on this server.
These websites do work when:
- The operating system is not a Linux distribution
or
- The browser is not Firefox
Example websites
While I normally wouldn't include a link to someone else's website, in this case I do because it is the website of a doctors office. These websites should be available to any patient at all times for anything short of a imminently life threatening emergency (in which case the national emergency number should be called of course) to provide contact information in times of need.
This website displays the symptoms described above: https://www.huisartsenpraktijkdehaan.nl/
There are more websites though, but the pattern is always the same.
The user-agent string
Trying to figure out what is actually causing this seems simple enough though. If I change the user-agent string to that of Chrome, it works.
So my tentative conclusion is that this is purely a user-agent driven bug/feature.
Some further testing yields this:
These work
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.150 Safari/537.36
Foo
Mozilla/5.0 (X11; Ubuntu; x86_64; rv:85.0) Gecko/20100101 Firefox/85.0
Mozilla/5.0 (X11; Ubuntu; inux x86_64; rv:85.0) Gecko/20100101 Firefox/85.0
Mozilla/5.0 (X11; Ubuntu; Linu x86_64; rv:85.0) Gecko/20100101 Firefox/85.0
Mozilla/5.0 (X11; Xubuntu; Linux x86_64; rv:85.0) Gecko/20100101 Firefox/85.0
X11; Ubuntu; Linu
X11;Ubuntu;Linux
11; Ubuntu; Linux
These do not work
Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:85.0) Gecko/20100101 Firefox/85.0
X11; Ubuntu; Linux
x11; ubuntu; linux
Hypothesis
Having the literal string X11; Ubuntu; Linux (case insensitive but including spaces and semi-colons as-is) in the user-agent HTTP header of your request triggers the broken behaviour.
The conundrum
I could, of course, reach out the owners of these websites (and eventually I will), but there is a catch. They likely won't use Firefox on Linux (because you would notice your own website being broke), and if they pass on the message to whoever maintains or built the website, the response may very well be “well it works for you, and it works for me, that user must have some weird virus-ridden computer and an ancient browser with a Bonzy Buddy toolbar”, or something similar.
So I want some more ammunition, and preferably a cause I can explain to anyone with a website like this. Even better would be to find out why this happens, and fix it at the source.
So what is happening here? Some Apache of Nginx module/config/plugin written by someone who really hates people who use Firefox on Linux? Some weird bug repeated on multiple sites?
Does anyone recognize this peculiar website behaviour?