Summary says it, when using the "Open containing folder" option in Firefox or Chromium, Thunar opens that folder, but doesn't scroll down and select the wanted file, so the user always has to scroll down to (and to find) the file, which can be extremely difficult with similar names and tons of files.
That is because they don't provide the file only the directory.
Note that unfortunately it's Xfce that ends up being penalised when applications don't do the right thing. Nowadays Firefox uses a new XDG "standard" (implemented by only *one* file manager) based on a D-Bus interface to show files in the file manager (causing Xfce users to have that file manager booting up instead of the Thunar, since xdg-open is no longer used). We could implement the D-Bus iface, since we probably already have all the functions it exports coded into Thunar.
Mozilla is currently investigating issues with their folder opening code. Once they have found how to reliably find the user's preferred file manager, they should be able to use Thunar's DBus interface and provide the requested feature. I'll update this report as I hear from them. Jannis, I'm stealing the report from you for now.
Marking duplicate. *** This bug has been marked as a duplicate of bug 11024 ***