Did you try to do a repair of avast? But then again i have doubts that it won't may not work because the link that you provided they had Firefox 26 and some had firefox 29. Firefox 32 is the latest version it should have been fixed? Did you try to post this issue in Firefox forum? I think it has got to do with Firefox. Also file a report about this to Avast Support Ticket via here
https://support.avast.com/ and give as much detail as possible. Since I am not using Firefox anymore and my younger brother is using it and he also has Avast installed I will ask him if he has this problem or not?
UPDATE: My younger brother's Firefox 32 and Avast Free 2014 is working fine! The link that you provided and on that link the user reported several issues listed below:
If i try to upload picture from certain websites, dialog (browse window) don't pop-up.
- If I select "Save Page As" from the menu in FireFox, nothing happens.
- If I go into options and specify "Download| Save files to" and click on "Browse", nothing happens.
- If I click on a file link (e.g. pdf, word) and I select "Save File", nothing happens.
- If I try "Export bookmarks to HTML", nothing happens.
- If I uninstall Avast, above issues are gone.
Those following issues are not present on his laptop. Perhaps a minority users are affected by this! Like I said before trying posting that to mozilla forum and Avast Support ticket. They will try solving it.