On my Mac (MacOS 10.11.6 "El Capitan", posting 8/12/21) I've used Thunderbird for years (decades?)--ever since Eudora was no longer supported--and this is the first time I've ever encountered this... I write an email, then click "Attach" and one of three things happens: 1. The browsable attachments window pops up (as usual, but no files or folders show within it and T-bird is "frozen," with just the 'please wait' beachball spinning endlessly. I have to Force Quit T-bird then start it again to no avail. 2. Same as above, but this time instead of the attachment window coming up, T-bird just crashes--quits itself almost immediately after I click the "Attach" button. 3. Same as "crash" described above in #2, but before it crashes completely up pops a window offering to let me submit a crash report. This happened once, but while going to look up the version number of my OS T-bird quit itself so I never had the chance to submit the report. Re-launched, clicked "Attach" again, and same as #1 above. In several more tries (re-launching T-bird each time) I never saw this crash report window again. ==>>On a hunch, I did do seem to have solved it (at least for now) by using Tools > Clear Recent History... , then checking all three items ("Browsing History," "Cookies," and "Cache") and "Time range to clear:" "Today." I'm posting this just in case someone else encounters the same problem and maybe the same fix will work for them. However, developers might want to take a look at this, as it shouldn't happen. If anyone has a better solution and/or explanation, I'd love to know what it is. I rely on Thunderbird--and sending attached files--and now I'm nervous. As part of my efforts to fix it, I did update Thunderbird to the latest version (78.13.0) before trying (and crashing/freezing) again.