Try with no-script disabled.

Resort to pen and paper, write down the email address, then reading from the piece of paper type it into the email address box on Waitrose's site.

Tools > Options > Content : JavaScript > Advanced > Allow Scripts to:
[] "Disable or replace context menus"

• http://kb.mozillazine.org/JavaScript#Advanced_JavaScript_settings
• http://kb.mozillazine.org/Prevent_websites_from_disabling_new_window_features

These answers do not solve the problem.

Example Site Affected:

https://www.waitrose.com/shop/InitiateLogin?

Tools > Options > Content : JavaScript > Advanced > All the "Allow Scripts to" entries are deselected.

Perhaps I should have put these details in the main body of the question instead of "More system details..."

I do get the context menu when left-clicking in the box, but the the paste function has no effect. (Allowing "Disable or replace context menus" kills the context menu altogether)

Do these Websites work in Firefox Safe Mode ??

Troubleshooting extensions and themes

• https://support.mozilla.com/en-US/kb/Troubleshooting%20extensions%20and%20themes

Clipboard not working

• http://kb.mozillazine.org/Clipboard_not_working

Check and tell if its working.

I hadn't checked the code and doing that I notice that there are attributes set to disable a lot of 'on' events for those fields.
You can remove them with this bookmarklet to make pasting work (and copy/cut).
It also enabled the context menu.

• http://en.wikipedia.org/wiki/Bookmarklet

javascript:(function(){var a=['oncopy','oncontextmenu','oncut','onpaste'],e=['logonIdField','logonPasswordField'],i,j;for(i=0;e[i];i++){if(E=document.getElementById(e[i])){for(j=0;A=a[j];j++){E.removeAttribute(A);}}}})();

Thank you! Your js bookmarklet solves the problem. This begs the question - why does the setting forbidding sites to "Disable or replace context menus" allow the context menu, but doesn't allow it to do anything (at least not paste). Perhaps FF4 has solved this?

You're welcome.

The setting about the context menu in Options only affects the context menu and not other 'on' events like done on this specific page. Other sites may use different code to do things like this and it is actually done most of the time by a script that runs globally on the page and not as in this case by adding them as attributes to the input elements.

I've discovered another way of enabling the paste function which may be easier to implement for most users. Install the FF Add-on RightToClick. It has a toolbar button to enable paste and works on the site I cited.