TimestampDiffs (source) adds a link at the end of every comment to the diff in which it was added.
The link is added to the timestamp that the MediaWiki software adds when you sign your name with ~~~~, and styled with a dotted blue underline. Occasionally multiple edits result in the same timestamp; in such cases the link will show the difference of all those edits to the version from before that time.
Even more infrequently, a link might not be found for some timestamps. The most likely explanation is that either it is from more than 25000 edits ago, or it was added manually instead of using ~~~~, or it was modified in a later edit.