OK.. first thanks Israel for offering support.
2nd.. I completely deleted firefox portable, and reinstalled from the installation file downloaded from portableapps.com. (version 6.01, on windows 7 with all the latest updates).
I then installed the lastpass addon directly from the link you provided above. It was a 2.4 mb download/installation, and appeared to run successfully.
However, as before, after a restart, there was no sign of the icon anywhere, no sign of lastpass having been installed in my addons manager, but one directory created in my extensions folder as outlined also above.
Out of curiousity, I then tried to install another simple no great frills addon, and although it appeared to install correctly, it clearly did not. Thus it appears I am left again with a firefox portable that is of little use to me as far as addons are concerned. - and yes, if I install firefox again, the 2nd addon installs perfectly fine, it is only after an attempt at lastpass installation, that addons somehow get their knickers in a twist.
any further suggestions?