Avast Firefox add-on slows down browser when blogging

Intel Xeon 8-Core, Windows 7 64 Bit, 8 GB Ram, Firefox 33.0
Avast Online Security 9.0.2021.112 (Firefox extension)

After publishing or updating a post on my blog (http://dansator.blogspot.com) Firefox freezes. There is a delay of about 25 seconds before the list of posts appears, and then there is a further delay of 10 seconds before Firefox reacts. If I disable the Avast Firefox extension the post list appears in less than a second and I can continue immediately. There is no apparent delay if the extension is enabled when I use any other blogger functions, such as viewing pages or archives.

I have an older computer on which I also use the same Avast extension (same version number). The delay after posting is less than 2 seconds.

Pentium Dual Core, Windows XP 32 Bit, 3 GB Ram, Firefox 23.0

Have any other bloggers experienced this problem? Is there a way to solve it? Is the Avast online security worth using anyway?