Can't reply to comments in Blogger?

You'll need to enable third party cookies.

Update:

In Google Chrome, you can add an exception for third party cookies in Privacy > Content settings, rather than allowing all.

In Firefox I found that it was Privacy Badger causing the problem by blocking the blogger.com cookies. Moving the slider to Allow fixed the problem.

I had already allowed third party cookies in Firefox, because blocking them caused a problem on a particular website (can't remember where), but it is also possible to block them and add exceptions in Preferences >Privacy > History > Use custom settings for history.

Update 2:

If you have blocked third party cookies in Firefox, add an exception for https://blogger.com:

Note: https

The CCleaner for Ubuntu?

CCleaner is an essential application for Windows. Ubuntu users occasionally ask about an equivalent for their operating system, usually to be told that it isn't necessary- Linux is not Windows.
Well, I think I've found the CCleaner for Ubuntu. It's called BleachBit, and on my computer it found 600MB of "junk": not an insignificant amount on a 10GB /home partition.