I found this while testing for bug 530403 on the Thunderbird counterpart.
If you enter the search dialog and switch the search term to "Age in days", not touching the default "is less than", you get a different search result than if explicitly selecting that option from the menu.
Steps to reproduce:
1. Search in a local folder with some messages less than n days old;
2. select "Age in days", default goes to "is less than";
3. enter a number n and click Search;
4. only a few results - if any - are shown (still looking for "is" instead?);
5. select "is more than" and go back to "is less than";
6. Search now shows the correct list of results.
The default was changed from "is" by bug 187741.
Some more testing (current nightly build), indeed the "is" filter is used when the "is less than" default is left unchanged (both IMAP and Local Folders).
Created attachment 413936 [details] [diff] [review]
So much for my original "fix" :-(
Pushed changeset 0ee062df1a25 to comm-central.
Pushed changeset 9bca5aa6d303 to releases/comm-1.9.1
*** Bug 529553 has been marked as a duplicate of this bug. ***