If you are searching for a comment on the system, you can easily search using the search functionality. 

On the left-hand menu side bar, click on Issues > Comments:

This will take you to a page where you can search through all of the comments which have been added to your system. There is a search bar at the top of the page, which you can use to search for a particular word or phrase:

There is a drop down which you can use to narrow down your filters:

The following filters will be opened up from this:

When you search for a word, only the comments with that word will appear in the search results:

This allows you to easily narrow down your search when looking for a particular comment. You can then click into the comment and this will take you directly to the 'Comments' tab of that issue.