I've been a long time Thunderbird user (except for the last few years). The global search feature was a good addition when it was launched, but it doesn't work very well and it doesn't seem like more effort was put into fixing/improving it. What I've observed is that using it to find specific emails may or may not work. I'm not talking about search performance but about the quality of results and the ability to narrow down. My mailbox was already a several GBs in size at that time.