Was that with the "intelligent streaming scan" option enabled or disabled?
Was Internet Explorer still loaded or already exited when you show that memory value?
UPDATE: Oops, didn't see firefox.exe (FF) was loaded. I only use iexplore.exe (IE).
How many web pages did you ever have at a maximum that were concurrently open in tabs? In my test, I went up to 24 (just because I had a favorites group with that many). Actually I got up to 42 concurrently loaded tabs but that was I doing an overview trying to find the cause rather than when I decided to test with the same suite of web sites with various config settings in Avast's Web Shield when it was running and also terminated. I don't know what Firefox has. Plus you need to do the test with Internet Explorer since that the web browser under test when ashWebSv.exe was noticed to retain memory. If there is a difference in how ashWebSv.exe handles its memory management between Firefox and Internet Explorer, and since the problem (here) is with Internet Explorer, you could be masking out the problem by not using the web browser with which ashWebSv.exe has problems. I haven't tested with FireFox.
Create a favorites test group in IE. The group that I used had the following web sites within it (4 were removed since testing because they no longer exist, like CastleCops):
http://www.wilderssecurity.com/forumdisplay.php?f=65http://forums.altiris.com/default.aspxhttp://forum.avast.com/http://www.daemon-tools.cc/dtcc/announcements.phphttp://boards.diynetwork.com/http://www.dslreports.com/http://dvdshrink.org/forum/viewforum.php?f=1http://www.fatwallet.com/http://forums.fedoraforum.org/index.php?http://groups.google.com/group/Google-Groups-Guidehttp://community.mcafee.com/http://www.wilderssecurity.com/forumdisplay.php?f=100http://forums.spybot.info/http://forum.spamcop.net/forums/http://www.spampalforums.org/phpBB2/index.phphttp://forum.storagecraft.com/Community/forums/http://www.bbs.legacycentral.org/http://forums.truecrypt.org/index.phphttp://communities.vmware.com/http://groups.yahoo.com/Open Task Manager and show its Processes tab. Sort the list by process name (so it doesn't jump around as CPU usage and memory consumption vary). Highlight the ashWebSv.exe process to watch it. With no web browser loaded, check the memory consumption by ashWebSv.exe. In the first round of testing, have the "intelligent streaming scan" option enabled.
Load the web browser but not to any page (i.e., use about:blank or about:tabs).
After creating the favorites group with all these shortcuts, click on the group and click the right-arrow for that group which then loads all shortcuts within a group. This will concurrently load the 20 web sites, each into its own tab. Wait until all tabs load their pages. You can use the Performance tab in Task Manager to see when network traffic ceases. Look at the memory used by ashWebSv.exe.
Right-click on the first tab (about:blank or about:tabs) and select to Close Other Tabs. You're left with just the blank tab. Look at the memory used by ashWebSv.exe.
Again click on the arrow for the group to have all web pages within it load into separate tabs. Check the memory used by ashWebSv.exe.
Right-click on the blank tab, Close Other tabs, and check the memory used by ashWebSv.exe. Load all the web pages for the group in separate tabs and check the memory used by ashWebSv.exe. Do this as many times as you want to monitor any memory growth by ashWebSv.exe, if it returns to its prior memory footprint when no pages are loaded, and if there appears to be some threshold to which memory is released but doesn't go all the way back down to its prior value when you first opened the web browser (with only a blank tab).
This testing has you check the memory footprint with pages loaded and with them removed. However, after you see the network traffic has ceased, another test would be to see if the memory gets released immediately thereafter since obviously there is no more traffic going throught Avast's HTTP proxy (Web Shield).
After bouncing around by loading a bunch of web pages and unloading them, and repeating this cycle for, say, 10 times, how much memory is used by ashWebSv.exe? With the web browser still loaded but displaying only a blank tab, does it ever return to it memory footprint when the browser was first opened (with a blank tab)? Does the memory footprint change (reduce) after you unload the web browser?
Then redo the testing with "intelligent streaming scan" disabled.