This problem has happened to me from time to time. You can always manage by opening a second tab as the others have said.
But for the lazy ones, there may be a solution to propose to
BPIP (if not already done). BPIP extension allows to quote a message from a locked topic, it could be done on all topics, locked or not.
Now we can do like this :
1-Open a second tab
2-Quote the concerned post
3-Select the concerned post
4-Copy the message
5-Switch tab
6-Paste the message where you reply
7-Switch tab
8-Back
[repeat actions 2 to 8 as many times as you want to quote a message]
9-Post
With BPIP for lazy people :
1-Open a second tab
2-Click on 'Quote to clipboard' on the concerned post
3-Switch tab
4-Paste the message where you reply
5-Switch tab
[repeat actions 2 to 5 as many times as you want to quote a message]
6- Post
The difference is not so obvious, but it would prevent the select and copy/paste problems.