I too experienced this issue. Just importing Firefox secrets didn't work for me either. I found a workaround.
nirsoft.net makes a free tool called passwordfox. It can read the Firefox password store and export a CSV file.
It can be found here: https://www.nirsoft.net/utils/passwordfox.html
Launch it on the computer and as the user you want to export.
Select option to add header info in passwordfox output with View -> "Add Header Line to CSV/Tab-Delimited File.
In Passwordfox hit ctrl a to select all and then File -> Save selected items.
Select Save as type Comma Delimited Text File and put the file on your desktop as ffpass.csv.
According to the Last pass help under Importing Passwords Lastpass understands a header line with any of the following column titles:
I exported my existing lastpass store as a export.csv on my desktop.
What it actually used for the export headers in my case was: url,username,password,extra,name,grouping,fav
I edited ffpass.csv in a spreadsheet, I used Libreoffice Calc, and deleted all columns except the ones below and updated the header as follows:
I added three more columns with titles extra,name,grouping
I left extra empty. It is the comment field.
I set the name field as appropriate and used grouping to put each entry in the right folder. I made all the folders before the import.
My final import file had these headers: url,username,password,extra,name,grouping
I saved my spreadsheet again as a CSV as fflogins_import.csv.
In Lastpass I opened my vault and went More Options -> Advanced -> Import and then selected Generic CSV File.
I loaded fflogins_import.csv in Notepad++ and copied the text into the import form and hit upload.
I validated the output and imported it all. Worked fine.