I can't replicate the problem on the Edgewall site.

As far as I can tell, the navigation links are generated on the server, so it should not matter what client software you're using, as long as the server gets the relevant cookie (necessary to keep track of previous and next for the current sort order).

Can you tell whether this is a style rule problem? To check whether the missing element is in the HTML, you could either:

(1) Right-click the Next Ticket link and choose Inspect Element (Q), then when the tree diagram appears, expand at the previous <li> element which should contain the Back to Query link. I have attached a screenshot illustrating.

(2) Select the navigation area, right-click, View Selection Source -- does Back to Query appear between the previous and next links?

I don't see the link in the HTML. I'm not sure how to tell if it's a style rule problem.

It's not a style rule problem, the server just is dropping that link from the navigation. But why?! I'm baffled.

Just in case it is a setting or add-on, could you do this three-minute experiment:

Create a new Firefox profile

A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should have completely fresh settings databases and a fresh cache folder.

Exit Firefox and start up in the Profile Manager using the system search box (or Run dialog) -- type or paste the following and press Enter:

firefox.exe -P

This should launch a small Profile Manager dialog. Don't delete anything here!

Any time you want to switch profiles, exit Firefox and return to this dialog.

Click the Create Profile button, assign a name like TEST06102016, and skip the option to relocate the profile folder. After creating the profile, select it and start Firefox in that profile.

Any difference on the trac site(s)?

When returning to the Profile Manager to resume using Firefox normally, you might be tempted to use the Delete Profile button. But... it's a bit too easy to accidentally delete your "real" profile, so I recommend resisting the temptation. If you do want to clean up later, I suggest making a backup of all your profiles first in case something were to go wrong.

I tried creating a new profile, but it doesn't help. This issue happens on several computers, all with rather minimal customization and plugins.

Did you use the list of tickets in the link in my first post? I notice it doesn't happen on all queries.

I tried a new profile and it definitely does NOT work for me in a new profile. I tried to spot the difference and I can't find it. My regular and new profiles are sending the same headers (referring page, cookies), and getting back different pages. The ticket in the new profile is generated in seeming complete unawareness of the query I used to to access it.

This doesn't make any sense, unless Firefox is actually not sending the headers it says it is sending. (This is difficult to detect with secure sites because these details are encrypted.)

Another PHP application, named My Little Forum, is having odd behavior in Firefox 47, maybe it's related: https://support.mozilla.org/questions/1127500 (unsolved).

Trac has a list of websites using it, some of them aren't encrypted.

https://trac.edgewall.org/wiki/TracUsers

I've found the same behavior on http://trac.buildbot.net/query?status=accepted&status=assigned&status=new&status=reopened&order=priority and http://trac.osgeo.org/grass/ticket/1444.

What are you doing to see what headers are being sent?

tmmunq said

What are you doing to see what headers are being sent?

Just Firefox's Network Monitor. I don't have a working external proxy (Fiddler isn't working for me right now).

https://developer.mozilla.org/docs/Tools/Network_Monitor