I can't seem to reproduce this issue here either. I also used:
//var %input = test | tokenize 32 %input | url -a http://www.google.co.uk/search?q= $+ $1
I tested the above with tabbed browsing and opened more than thirty windows in IE each time.
If you test this with a clean install of mIRC with no scripts etc. does the issue still occur?
Under which version of Windows is this happening?
If you close mIRC and then open up several browsing tabs in IE and then try to run other applications, eg. Word, do you see the same issue?