In Vimium, when I want to copy-paste text, I search the string with /mystring ENTER. Then I go into visual mode with v, where I can move around with h,j,k,l keys. However, when the search ends up at the middle of a string, I'm in trouble, because I want to copy the whole string, and the copying starts from the middle of the string.
How can I move left to the beginning of the string, without highlighting the beginning of the word?
After you go into visual mode with v, use c to enter caret mode. That mode allows you to move without highlighting. After you've moved to the correct position, press v again, and continue editing in visual mode.
Additionally, if you want to copy whole words, then while in visual mode, you could use w. That highlights an entire word. And if you press it more than once, you get more than one word.