Manual input from user while running selenium IDE script

In Selenium IDE 2.8.0: Capture the test session entering the login and password. Edit the Command for each and change from "type" to "waitForText" and remove the values in the Value fields previously stored for login and password. Re-run the saved test script from the IDE.

You can use the following script to invoke a javascript prompt in order to get the value

<tr>
    <td>storeEval</td>
    <td>prompt(&quot;Please enter your FirstName&quot;)</td>
    <td>firstName</td>
</tr>

Then accessing the value is a case of using ${firstName}.

EDIT: Since Selenium IDE 2.5.0, the script needs to look like this:

<tr>
    <td>storeEval</td>
    <td>javascript{prompt(&quot;Please enter your FirstName&quot;)}</td>
    <td>firstName</td>
</tr>

As of 2020 using Selenium IDE 3.16.1, I found the following config worked by the suggested answers above. (I re-answered the question as above answers did not clearly combined all pieces)

Command : execute script

Target : return prompt("Question Text","Default value");

Value : yourVariableName

and fill in the input box by the stored var:

Command : type

Target : (your selector)

Value : ${yourVariableName}