Check out the Enable All Text Selection bookmarklet by Alan Hogan. The only issue with the bookmarklet is that it does not handle frames/iframes (that’s a browser security thing so it’s unlikely something can be done about it).
As an added bonus, it also enables the mouse right-click event on pages that block it.
Create a bookmark (e.g. by dragging the icon to the left of the URL for any page to your bookmarks bar), right-click and select Edit, rename to something meaningful, and insert the following code in the URL field:
javascript:(function(){function%20allowTextSelection(){window.console&&console.log('allowTextSelection');var%20style=document.createElement('style');style.type="text/css";style.innerHTML='*,p,div{user-select:text%20!important;-moz-user-select:text%20!important;-webkit-user-select:text%20!important;}';document.head.appendChild(style);var%20elArray=document.body.getElementsByTagName('*');for(var%20i=0;i<elArray.length;i++){var%20el=elArray[i];el.onselectstart=el.ondragstart=el.ondrag=el.oncontextmenu=el.onmousedown=el.onmouseup=function(){return%20true};if(el%20instanceof%20HTMLInputElement&&['text','password','email','number','tel','url'].indexOf(el.type.toLowerCase())>-1){el.removeAttribute('disabled');el.onkeydown=el.onkeyup=function(){return%20true};}}}allowTextSelection();})();
To make the bookmarklet code readable you can use the Bookmarkelt Builder at http://subsimple.com/bookmarklets/jsbuilder.htm – just paste the minified bookmarklet text and click the Format button.