Taking a look at the source code (for Icecat 9.0.1), the URL you give does not appear to be present anywhere in the code explicitly. In fact, the string "icecat/addons/" does not appear anywhere in the source code.
Given the fact that the incorrect URL does not include this string, though, it doesn't make much sense to be search for it. Instead, it would make more sense to search for "/icecat/themes/".
To answer the question of where the incorrect URL may be in the source code, the file ./browser/app/firefox.js includes the line pref("extensions.getMoreThemesURL", "http://www.gnu.org/software/gnuzilla/addons.html#themes");
(on line 203). This is the correct URL for the themes page, right? If you are working with an older version of the source code, I would expect that the problem lies in that file. The file ./testing/mozmill/tests/shared-modules also contains the line , so that may also be of interest.