User-Agent: Mozilla/5.0 (X11; Linux i686; rv:2.0.1) Gecko/20100101 Firefox/4.0.1 Build Identifier: Mozilla/5.0 (X11; Linux i686; rv:2.0.1) Gecko/20100101 Firefox/4.0.1 Several times Firefox would hang when I tried to edit the cache size in the option dialog, and then when it came back most of the cache would be gone. I can now reproduce this. It appears to be due to instantApply (Linux default, but probably on other platforms if they set instantApply) thinking the cache size has reduced when a digit is being overwritten, but before the new digit is detected. Reproducible: Always Steps to Reproduce: 1. Check in about cache that the disk cache is nearly at the configured limit 2. Make sure browser.preferences.instantApply is true 3. Navigate to the cache settings in options/preferences 4. Select override automatic cache management 5. Highlight all or a portion of the existing number and overwrite it with a larger number Actual Results: 1. Browser hangs for a while, a few seconds or quite a lot of seconds (cache is being purged) 2. Check about:cache again and see it is much less than before although the maximum storage limit is increased Expected Results: 1. Browser shouldn't hang because cache shouldn't be purged 2. about:cache should show cache usage unchanged and a new higher maximum storage size Note that there are ways to increase the cache size without this happening. The obvious way is the scroll wheels but this isn't always practical. It is sometimes possible to edit the number in such a way that the digits visible never represent a smaller number, but not always. For example, typing an extra digit after "250" will cause Firefox to edit the number to "1024" and in the process the cache gets truncated severely.